I found the following explanation using google (google is your friend):
Q: One day, my Local Disk hard drive reported that I had 12GB of free
space. A few days later, it decreased to about 8GB. Today, I added a few more
files, but instead of reporting less free space, it now shows 9GB free. This
constantly fluctuating hard drive space has been happening a lot to my drive
that holds Vista, but it never happens to my storage drive. What's going on?
A: This could be due to the c
agefile.sys or c:hiberfil.sys that can take
up several gigabytes of space depending on the amount of RAM you have on your
system. Crash Dumps can take up some space, too.
More than likely, it's System Restore creating restore points. Every time
you get a Windows Update, even if it's just an update for Windows Defender
(those restore points can be stopped) a restore point is created. Most of the
time, when you install or even uninstall a program, a restore point is
created. When the restore points get to a certain size, some of them are
deleted. If this is a new installation or new computer and you are setting it
up, installing software, you are going to see this type of free space
fluctuations. You can use Disk Cleanup to manually delete all but the last
restore point if you'd like.
> hi. every couple of times im on my computer i glance at how much free space i
> have in my hard drive. it changes a lot. about four days ago i decided that
> id make a note of how much free space i had left as i was getting sick of the
> fluctuations. the tile view in computer told me i had 20.0 gb left, which was
> about how much i figured i should. then, two days ago it told me i had 11 gb
> left which was odd becasue i hadnt downloaded or copied on 9 gig worth of
> stuff over those two days. then last night i got a warning telling me that it
> was just about full, with 1.7 mb left. so i freaked out and turned off the
> internet becuase i had downloads and whatnot and proceded to burn some of my
> videos to dvds. i had copied off 5.7 gb and i checked to see how much i had
> free and it told me 17 gb. and now, as i write this it says theres 23 gb
> free. what the hell is going on? if it isnt fibbing to me about how much
> space i have on it, then how the hell could 20 gig just sneak in without me
> knowing? any help greatly appreciated, tristan.