Ok, in my case, it seems that CPU was overheating, I've never overclocked it by the way.
I have AMD Athlon 64 X2 Dual Core 6000+ and after I changed it from 3000MHz to 2800MHz in BIOS and disabled Cool'n'Quiet (in BIOS too) I have zero crashes!
Yesterday I had one every 5-15 minutes (no rebooting, just CTD).

I have the same issue. After maybe 20 minutes a bit of stutter followed by complete shut down. Only happens in witcher 2.
my specs are
amd x2 250 not OC'd
6GB of ddr2 ram
ati hd4850
win7 64 bit

What? Can overheating cause those normal CTD's (you know, the game just drops and shows the launcher, and no error message?)

I was under the impression that overheating reboots the whole machine..

One possible explanation is that my CPU started to generate errors in higher than normal temperature (but below critical temperature) when it was 3 GHz so reducing it to 2.8GHz made it more stable.

That's how manufacturers label CPUs - if the same unit doesn't produce errors when set to 3 GHz than sell it as 3 GHz otherwise try lower frequency, etc.

If it is the case, it seems that my CPU was labeled 3 GHz quite optimistically...

Getting the same thing. Reboot (ie. not BSOD) at end of some conversations.

System at around recommended level and no OC'ing.
Temperatures and voltages nominal.
Everything updated, including BIOS.
Have tried WHQL drivers as well as latest Beta.
All settings off or low, windowed and fullscreen both.
All drives defragmented and memory tested.

Getting the Windows SDK debugging tools now to see whats what. Considering how touchy the game is in regards to drivers though I willing to bet its a coding error somewhere.

@Veshta: Please keep us updated if ya do find anything!

I can monitor/check my hardware all day, but when it comes to finding out any kind of software/coding issue I really am a fish out of water.

All I know is my hardware is performing the same as it does in everything else I run. And from what I can make of it, on a hardware level, there is nothing out of order.

It's ignoring BIOS settings, I'm getting no dump file whatsoever to make heads or tails, or at least give to someone who could, of what is going wrong.

Same boat, temps, voltages, Nvidia drivers, chipset drivers, BIOS, low settings, high settings, windowed, fullscreen, daily scheduled defrag... Hitting on all cylinders, yet I'm still getting this system reboot -- seemingly only in the menus.

Hell, the first time I tried low settings I got the cloudy yellow image and before a sound was made it rebooted. But as best I can recall I've not gotten any problems while in in-game cinematics, or during gameplay.

as Ohtheknives i have only the reboot/crash/freeze happening in the menu such as the inventory and as everybody, I checked everything in my system which is a very recent top notch gaming computer. Not heating, no CPU/CPU problem etc...the game run very smoothly even with everything at the max setting but keep on randomly freezing, quit, reboot in the menus such as the inventory screen :( eve at the lowest setting.
I mentioned this as I think more people mention this weird thing more attention we can get because even with these flaws I love this game.

I am at a loss.

Read on steam forums that it might be related to HDD's powering down, changed all drives to never power off, and I was able to complete the game with a couple of "normal" CTDs as the only major faults.
Second playthrough the reboots starting occurring again with no changes made in the meantime .. system updated, cool and PSU rated at roughly twice the component requirements (850W).

Crash is probably motherboard related (the last thing possible) since it essentially hits the reset button, ie. no crash logs are ever made by Windoze, and as far as I know the MoBo is the only component with power control of the whole system.
Queer thing is that it has not happened in any other game and I regularly run several at the same time (Eve Online x2 + Civ4 or Civ5) while watching movies are listening to music on the side so error must be Witcher2 based but what/why they could want to access power management for is beyond me :D

PS: Patched to 1.1 when it became available and it is still happening.

@Veshta: Thanks for the input! :)

I've also read about the HDDs powering down, but it hasn't seemed to work for anyone that I've noticed, so I haven't bothered. Really haven't bothered the last few days, with a holiday weekend and all.

Patch 1.1 did not fix it for me, either.

Still, oddly enough, the only time it happens is when I'm in a menu of some sort. How do they occur for you? During gameplay/cutscenes? Or only in menus?

I've had very few hard freezes during the splash intro movies, and once during gameplay I believe, but it only entirely reboots itself when I'm in a menu.

I had, especially after patch 1.1, a LOT of these random reboots. It made me quite mad, it was beyond the limit of unplayability.

Then I tried some tweaks (using RivaTuner to force the GPU fan to 100%, etc.) with no results.

In the end, at least for me, I found out what was the problem: SSAO enabled, as soon as I disabled JUST this option, the random reboots were gone and a memory of the past.

I have all the other settings at ultra (except obv Ubersampling, and I completely disabled all the DOF settings, in game and in cinematics), and it runs perfectly fine now.

Try disabling the SSAO, it worked for me (Win7 Ultimate x64, i7 920, GTX275 with latest nVidia beta drivers, 275.xx).

Can anyone confirm this works? Am experiencing lots of quits-to-desktop and BSODs.

Well, you can confirm it (or not) by yourself, just launch the game with SSAO disabled and look if it still crashes. ;)

I think your machine is doing what it's suppose to do!! ..a crash and a proper re-boot!! Good for You!! Oh, the game caused it....shucks..bad game!! (buy GOG's version...)

@Galandil:

Did you monitor temps and such on your GPU before disabling SSAO? Were they having any heat issues? Or was it all everyday occurance, then random reboots?

I've tried running it on Low settings, which I'm sure disables SSAO, and I still get these reboots, so I'm not sure having everything on Ultra (minus Ubersampling and SSAO) would have any effect.

As stated, it's only in the menus when it reboots. Nothing out of the ordinary. No go-to culprit outside of the generic "faulty hardware," when it almost painfully obviously does not appear to be the case.

Yes, I checked out the GPU temps with RivaTuner, even tried to force out the fan to run at 100% before launching the game while keeping the case open, and nothing changed, still a lot of random reboots.

I was going to check with another heavy game, like Crysis, but then I tried to set everything on Ultra, and disabling Antialiasing + SSAO (I always kept Ubersampling + all three options for Depth of Field disable) from the options, and I got 0 crashes. Then I added AA (no crashes) and then only SSAO, and I got again the crashes, so, at least for my rig, it must have been that option.

I have to mention another strange thing: the test application returns 0 MB of memory for my GTX275, thus setting everything at Low settings. Really strange since I've 768MB of VRAM on the graphic card.