Check out how much space is used for "System Restore" if you're in Win XP or ME (control panel - system). Also, are you using a laptop and making files from another computer available offline? They're hidden in \winnt\csc on my windows 2000 machine - check the size of this directory if you have it. Saving internet pages for offline browsing can be another space hog. Lastly, are you sure you don't have any CD or DVD images lying around on your drive
?