Yeah, it'd either be a swapfile, tmp files (including recovered disk errors) or upgrade files.
Have a look inside
C:\Documents and settings\(username)\Local Settings\Temp
and
C:\Windows\Temp
and see if there's a fuckload of stuff. Some stuff can't be deleted 'cos it's gonna be in use by XP but most stuff can be safely trashed.
Look around in the root directories of your hard drives for .chk files too - they're recovered files from CHKDSK scans and can usually be deleted too.
The swapfile... well, you're shit out of luck with that if it's big, it's kind of necessary for a lot of stuff (even when you have a shitload of RAM too - some programs actually rely on that to work at all).
Upgrade files... go into the Add/Remove Programs and see if you can find uninstall options for the upgrades - it'll usually tell you if it's safe to uninstall them.
... or then again, it could just be the porn too!