On a side note, do you have Indexing Service turned on? If so, you might want to turn it off. You might also want to delete all of the files contained in c:\windows\prefetch. This may or may not free up a lot of space, but worth a try I guess.

Also make sure to delete the temp folder files for each account (guests, admin, etc).

You could also try doing a search through start>search>for files or folders, and then having it search for all large files/folders.