What you might need to do is run a disk cleaning utility, windows has one...accessories, system tools, disk clean up. Also try a defragment on the drive with windows defragmenter. Many times a fragmented drive will read almost full.
Also, clean history files,cache etc...
download ccleaner and run it, it is easy and does a great job, and free. It will clean the history, junk etc...also it has a registry cleaner\fixer that you can backup before cleaning.
also run ad aware since you may be cluttered in with spyware and don't even know it.
Give these a try first and see what happens.
I have a fairly new machine running Win XP. A few weeks ago, I had approx. 40% of 'Free space' on the hard drive. Now, it's 95% full. So, I went through all the folders and have 2.87 Gb in Windows, 3.33 Gb in Program Files, 2.7 Gb in My Documents, and a few megabytles here and there. If I did my math correctly, it doesn't add up to fill a 30 Gb hd. I ran McAfee and came up with 'no infected files'. I deleted all restore points except the most recent. Haven't figured this one out yet. Any clues?