I've got a Windows XP Computer with a C drive that's very nearly full. I've tried to defragment. However, this process won't work--I get a message saying there's not enough free space to run a Defrag, since it requires a certain amount of free space to function. I have transferred all my iTunes, photos, and documents to an external hard drive, but the C drive is still too full and I think it's slowing down my system (slow to start up, frequent stalling out, etc.) I don't want to wipe it completely clean. I know to frequently empty the temp files, etc. and have used the Disk Cleaner utility but nothing seems to help. I've also uninstalled a lot of programs. How can I know what else is safe to delete? Is there a better way for me to purge whatever's filling it up?

You might be a victim of Brontok virus, which replicates files in the computer. Including system files which a hidden. Get a good anti virus program and a registry clean up program could help too.

First place to start and then go from there to see how much space you can get back

-Right click on C: drive in windows explorer and choose properties
-Click Disk Cleanup
-When it gets done searching, click on the tab "More Options"
-Click Clean up next to the System Restore and say OK
(This will delete system restore points except for the last one made)

By default, system restore is set to save up to 12% of your hard drive space just for restore points - So 100gb hard drive is set to save up to 12 gb by default