The amount of free space on my hard drive keeps changing and I'm not sure why. This happens without me downloading or deleting anything.

Also, sometimes my cpu is being used at a higher percentage than any of the programs in the program list can add up to.

can be anything, like windows updating on the background or the virtual memory it is using.

and you mean the non active system processes at about 90%? that's normal.

Volume Shadow Copy? It is one possibility, as are the restore points. Also possible, some temporary files, though depending on how much the free space changes by, it might be nothing significant.

It could also be Windows making some restore points.

it usually fluctuates by a couple gigs. Maybe it is just nothing. Never noticed it before I guess.

If you have your virtual memory set to a variable amount, try setting it to a fixed amount, and see if that influences anything. Virtual memory is the most likely cause for continuing fluctuations in the range of a couple of gigabytes, imho.

Some time ago my HD shifted from 73GB of free space to 27(!) in two days. I was kind of worried at first, but then I read it was probably Windows making a restore point.
The weird thing is that after a defrag the free space somehow jumped to 94GB, wich was more than I had before. Since then it has been slowly dropping by a gig or two a day, and now it seems to have stabilised at 64.

Page file size, or most of the other mentioned stuff. Unless your drive is a smaller drive (like a SSD or something) it's not worth getting worried over.

Also, even if you don't consider yourself to be "downloading anything", Firefox could still be prefetching a Flash video and caching it, for example.

Or any number of programs, including the OS itself, are running an auto-update check. A ton of possibilities. Nothing to worry about unless the PC is using significantly more and more drive space without you making any major changes. And even then this might be normal for some folks, depending on what they have running and various OS settings.

Probably Page files. For instance, a game like Rage has a 1 gig page file on my hard drive and I've only played it for an hourish. Start a bunch of different games and they can add up.

When I defrag my computer, I lose free space. But I fixed that by regularly deleting shadows copies. However, it does not fix the problem that the free space keeps dwindling down. It can fix the issue temporarily/slow the process down.

Go to My Computer
Right click the main drive(most likely C)
Properties
Disk Cleanup
Choose either one (Better is all users)
More Options tab
Cleanup for shadow copies - It will also delete all restore points except the latest one

This is on my Vista computer.

If you want to turn off automatic system restore creation on Vista then open the Control Panel and open the System option. At the left hand side select System Protection. You then get the System Properties dialog box open up at the System Protection tab. The lower half off this window has a tick box for turning on/off automatic restore points.