The HP support forums give no good suggestions and the official HP support (phone or chat) seem unavailable, so here goes:

Notebook Model: HP Pavilion Sleekbook 14 PC
BIOS Version: F. 18
BIOS Vendor: Insyde
Windows 8 64bit Single Language (preloaded on this PC)
No CD nor DVD drive (Ultrabook).

This is a notebook of a friend. At some point he reported it just shows a black screen with the mouse cursor after booting it (after it loads Windows 8). He has no idea what happened before that, I read many reports online people having issues after e.g. some broken Microsoft updates for Windows 8 or 8.1.

He is fine with the idea of "clean install", nothing needs to be backed up. Recovery media (USB stick) was created when the laptop was taken into use. I boot the notebook with the recovery media and get to HP Recovery Manager. I select "Factory Reset" which is supposed to erase the whole hard drive and restore it to the original factory condition.

After 30 minutes or so, it has completed the recovery preparations, which I presume were re-creating the recovery partition to the hard drive etc. It says the PC will boot itself several times during the recovery process, and I should not meddle. Ok then. After I let it continue, the notebook boots up, HP logo and the rotating circle is shown. After awhile I see a glimpse of what seems like an empty Windows desktop, and one dialog window which says something like:

HP Notebook
<date> <time> (e.g. 8/14/2015 11:15AM)
Please wait...
(there's an ok button in there I think)

That is displayed for a fraction of a second only, after which the notebook just reboots itself, and does that same over and over again, showing that dialog screen each time after the HP boot up logo. I waited around 8 hours, and it just kept doing that same, so it seems the recovery process is interrupted each time with a reboot.

So, what next? I have resetted BIOS the default values just in case (albeit I think nothing was ever changed anyway), no effect. I haven't tried to update the BIOS yet even though I located a newer BIOS/UEFI for this notebook, I'd rather try that as a last resort, and/or after I have the PC up and running.

I also tried the other, lighter, recovery options ("System Recovery" and "Minimized Image Recovery") before "Factory Reset", but they didn't get even as far, I think. Also, I have run a full system checkup from the start menu (RAM and hard drive), everything was reported as fine.

Is it possible to try a clean install with a bootable Windows 8 USB media? Where can I obtain such a media, and does it work also with Single Language edition?

Try installing from USB stick. http://www.howtogeek.com/186775/how-to-download-windows-7-8-and-8.1-installation-media-legally/

My best experiences with resetting an OS involve the OS on a disc which includes everything for the base install (including drivers) and might even include the cd-key if it matches the hardware configuration it's expecting, so you'd just stick the CD in and let it go.

Unfortunately most companies don't include the base install anymore, you might be best to do a raw backup of the drive when you first get it as a reset if need be... But naturally that wouldn't be the case :P

I'm not sure what's all on the hidden partitions, if they include the full base OS or just emergency recovery tools, they never made sense and were never explained, so i wouldn't rely on them.

You could maybe take the hard drive out and do a base install on another computer, then once it goes to reboot the first time after copying the files before doing hardware detection, move the drive to the laptop, at which point the remainder of the install will go cleanly without relying on CD or USB stick. I did that on my gaming rig which lacks a DVD/CD Drive.

I'd recommend putting Win7 on instead, but that's just my opinion (although obviously the key won't work)... Or try going free-OS with Slax, Debian, Ubuntu or something.

I'd be fine with it, if it just worked. Now it doesn't, but it doesn't give any error either, e.g. indicating that the recovery partition and/or media is somehow corrupted.

I'll probably try to create some kind of Windows 8 core USB install media if I am able to clean install that way, but I fear the fact this is a cheaper SL version of Windows 8 somehow prevents that too. This is all quite confusing... From the HP forums I also get suggestions that I shouldn't do that because it destroys the recovery partition and then I am really fjucked up etc. etc. etc. etc.

I'd also like to update the BIOS/UEFI, but I seem to have so much bad luck with this particular notebook that it would probably brick the system as well. This notebook came with Windows 8 (SL), so I rather want to try to restore it, and then see where I can go from there, e.g. updating it to 8.1, and maybe even Windows 10, in case Single Language Windows 8.1 is also eligible for the free Windows 10 update (I have absolutely no idea, damn Microsoft with their various Windows editions...).

I'm thinking of installing Linux Mint on it just to confirm that the PC works and it is the Windows 8 installation/recovery which is the culprit here (PC diagnostics didn't find any problems in RAM nor hard drive), but the owner probably wants Windows back to it. I'm not ready to convert him to a Linux user quite yet.

If you still have a recovery partition you should be able to restore factory installation with F11 key during bootup.

http://h10032.www1.hp.com/ctg/Manual/c03601748.pdf Restore and recovery

I'm reminded that i was asked to restore a computer which had Windows Media Center version (or something like that), some oddball version of windows i'd never heard of before and required 3 CDs to install, only 2 of which i could find/download; When it turned out it wouldn't work properly i ended up installing XP and calling it good...

I don't know if I have it and whether it is healthy, but I do have the recovery USB media which was created when the system was first taken into use. I can initiate three different kinds of recoveries from it:

System Recovery
Minimized Image Recovery
Factory Reset

All of these seem to end into the same problem: after the recovery preparations are done and the system boots itself up for the first time to start the last parts of the recovery process, I end up into the endless reboot cycle.

Also, when the system is booting up, I can push Esc to get to boot options, one of which is indeed system recovery (F11) if I recall right. Selecting that seems to just go to that same endless reboot cycle right away.

So, it just seems the recovery is just not working right on this machine for some reason, either with the recovery partition nor the USB recovery media. If only I knew why the PC decides to reboot itself right when it is supposed to continue with the recovery... I know Windows 8 was supposed to be smart in how it reboots itself on system errors, but I'd really love it if it would give me, the user, some feedback. Why does it reboot? What is the error? Don't hide information from me.

I was suspecting the hard drive would somehow be faulty, but I've already inspected it with various tools, all say everything ok (SMART, deep diagnostic scans, chkdsk /R c: etc...).

Can't pretend, I'm an expert, but - this reminds me of the very same situation, that I once had.

The solution was to remove the battery (accu?), remove the power-cable, too and then press the "on"- button for around 10 seconds.

After that, it worked again.

From my personal experience repairing something broken usually takes much more time than replacing with new ; ) You should ask yourself if it's worth of your time trying to repair this particular installation. Have not much experience with Win8 to be honest, afraid I can't be of much help with it's errors.. Still using Win7 on main and XP on an ancient netbook.

The downloaded "Windows8-Setup.exe" asks for a product key when I run it (in order to create an installation media). Any idea how to obtain that from the other PC? Windows 8 (SL) was preloaded on that notebook, and from what I've read elsewhere, on preloaded Windows 8/8.1 PCs the product key is embedded into the BIOS/UEFI. No idea if it can be seen there somehow.

I suspect that Windows 8/8.1 media creation tool is meant only for people who bought a Windows 8/8.1 license (or upgrade) separately, and it can't be used for preloaded Windows 8.x systems?

I have no idea why installing Windows has to be so cumbersome nowadays. Is Microsoft making it a real pain in the back purposely? Everything works fine in theory, but exceptions to exceptions to exceptions... "Oh, your Windows was preloaded, and/or a bit different edition! Then forget about these instructions, they don't apply to you at all". FFS, Microsoft...

Again I'm starting to get miffed at Microsoft. Linux installations feel like a walk in the park compared to all this hassle and jumping through the hoops. Why did the HP notebook restrict recovery media creation only to one time? Why doesn't that precious one recovery media work now when I actually need it? Why can't Microsoft provide a simple reinstallation media/tool that works for all situations? Am I supposed to buy a new license from Microsoft, or order new recovery media from HP? What if those don't work either, and the problem is somewhere else? I actually did that yesterday, read somewhere that is the way to do a "hard reset". But it didn't help any. If the PC is broken, then I guess yes. If e.g. the hard drive was broken, then I'd simply replace it and reinstall Windows 8. Too bad so far all diagnostics say the hard drive is fine, but I'm thinking of e.g. installing Linux on it and run some heavy-duty diagnostics with it.

If it is the Windows installation which is somehow broken, then I'd expect Microsoft to let me reinstall it from scratch. Now I feel as if they are trying to prevent me from doing that and continue using the product I've paid for, for some reason. Why does it have to be so overly complicated? Why all the hoops?

This link can be helpful: https://answers.microsoft.com/en-us/windows/forum/windows_8-windows_install/where-do-i-find-the-windows-8-product-key-when-it/d4c5c0c1-825d-47f2-9bed-d9625c7e68ff?auth=1

edit:
or go linux : ) running linux from usb is way easier than windows, you can get any iso and put in on usb stick/hdd.
Plnety of tools available..
RMPrepUSB
UNetbootin

Thanks, I think I read that page too yesterday (I feel I have read dozens of web pages online, trying to seek help for this problem...). I didn't read all the 16 pages of it though, not sure if there was some important data hidden there somewhere. Just read the replies marked as solutions (which don't appear to be solutions for my case).

The selected reply suggests some tools I am apparently supposed to install while in Windows 8, but the problem is I don't have a working Windows 8 installation now.

The second reply from the same person suggests that that product key is not really needed by the user anymore on preloaded systems, as it is embedded into the BIOS. Ok then, but what do I tell that Windows8-Setup.exe which requires me to enter it in order to proceed with the media creation? Microsoft...

The only next step I can think of is to wipe the whole hard drive (all partitions) manually away somehow, and then retry the Factory Default recovery (hoping the recovery partition will be re-created correctly). I'm starting to suspect there might be something wrong with the current recovery partition, and the Factory Default doesn't currently recreate it at all but skips that step, apparently assuming the partition is there already and nothing needs to be done (because it goes over that step so fast, I have hard time believing it is able to copy the whole contents of the recovery partition from the USB flash stick to the hard drive in a couple of seconds).

You should be able to do that by:

-go into setting
-go under update
-you should see a backup voice or something similar, select that

Now you can choose to do a fresh install, or install Win 8 again but manteining your files.

This way you don't even need your cd-key, since that is stored into your UEFI.

Personally I'm using modified Hiren's Boot for ages in such a case. You could try Recover My Password Home Edition or http://www.nextofwindows.com/how-to-retrieve-windows-8-oem-product-key-from-bios for example.