Have you ever tried to move, delete, or perform other operations on a file or folder only to be notified that it is locked because it is being used by another process? Enter freeware Unlocker to the rescue. This software can (a) identify the process(es) that are using your files/folders and holding them hostage, and (b) will let you "unlock" these or optionally to kill the offending process so its no longer running.
Unlocker installs a convenient entry in the Windows context menu that can be used to right click any file or folder and investigate the processes that might be working with it, if any.
Works under XP, Vista and its free.