Vista "Unknown" 5gigs in C Partition

Hey, thanks for your help.

I have my Windows Ultimate 64 bit on a 30 gig C partition, and for some reason it's almost full.

This happened all of the sudden of course, as far as I can tell with no provocation. Using WinDirStat I found that I have 4.6GB of "Unknown" files. I checked my ghost storage and I only have around 2GB allocated for it.

My user account uses another 4.9GB, mainly in downloads, but I know about that so I can deal with it (I'm going to transfer then to my other partition and stop downloading to C). Windows, and this strikes me as odd, uses 15GB of storage. Especially since I only have 2GB DDR1066 and my paging file is only 1.5. I've just concluded that this is because Vista sucks and nothing more, just because I can't think of any different reasons.

This huge unknown file is what's getting me. Is it something to do with WinDirStat? Right now I'm defragging, although I'm pretty certain this won't do anything. It's also not a virus as far as I can tell or a decompression bomb or anything of the sort.

Oh, and they aren't old compressed files or anything that Disk Cleanup would get rid of.

Any ideas?

That's unknown too. The help file told me that System Volume Information sometimes hid some information from them, namely system restore files. I cleared them though and it's still there.