File is in use... By what?

Do any of you guys know of any programs or methods of figuring out what programs/processes are currently using a file/folder, preventing me from renaming or deleting it?

My stupid laptop has been doing that to folders for no apparent reason. I’ve been looking for viruses/spyware and I’ve checked the processes list but I don’t see any likely culprits.

It’s really rather annoying. I have to restart my computer to be able to rename or delete the files.

The OS is Windows XP SP2… Any ideas?