Don't mean to scare you, but that sounds like a virus to me. Viruses sometimes make your computer believe it's full (or sometimes make it full by filling it with "junk" or "filler" files to slow it down).
Before deleting anything (sometimes deletion of the wrong file can lead to the "birth" of new ones... just wiki "spyware" to see what I mean) I suggest running Ad-Aware, Spybot-Search&Destroy or any other good spyware detectors.
Also, if you can muster the space, run Dr. Webb's CUREIT software. (
ftp://ftp.drweb.com/pub/drweb/cureit/cureit.exe) It's an always up-to-date, fast, and most importantly FREE anti-virus solution.
If your computer is saying you don't have the space, load up a flash drive and run the antivirus/spyware stuff from there.
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Aside from virus/spyware infestation, another good way to get space in a computer that's saying it's had enough to eat is to run Windows' Uninstall package. Start -> Control Panel -> Add/Remove Software. Remove the software you don't want, get back your space.
And of course, as others have said make sure you delete Temp files (Internet AND Operating System), Recycle Bin and unused software.
Good luck, hope that helps.