I guess you never shut down? I don't think XP would allow you to boot with 0 space left, may be wrong though.
I assume you've cleared out all temporary files? search for *.tmp also check the temporary file directory.
Clear out internet temporary files (cache) - you can do this via your browser.
I don't use Windows so can't check, but if I remember correctly, there is another temp folder within Documents & Settings under your username. You might need to enable hidden files to see the folder.
Then you need to work out where all your space is going! You said you've done virus scans, what about spyware/malware?
As mentioned above - check the size of your paging file. If you never shutdown it could well have gone a bit mad, in which case a reboot should reset it.