CCleaner (Crap Cleaner)
Cleans pretty much everything from your computer. Temps, cache, history, even has a registry cleaner that can get rid of a lot of errors.
Highly recommended. And its safe, the PC repair shop I work at uses it now, as do I and a few of my friends
One thing to note: If you have more than one user profile (logon) in windows XP you will need to run it on each profile to delete things from their respective temp folders.