Have you tried cleaning out the cache, internet files, history, temp folders, etc. Run a scan to make sure no spyware has been installed, sometimes it can chew up space. Make sure there aren't any hidden files, large attachments in the mail, just basic things like that. You might even want to try compressing the disk. After removing a bunch of programs or stuff in general, the disk might need to reorganize itself. (not sure, but stranger things have happened.)