Sysinternals.com has a great little 18KB tool for those odd times in which you find yourself unable to move, delete or rename a file due to a “file in use” error.
Handle is Sysinternal’s command-line utility freely available for download that will display what file handles are open on your system and to what process they belong to.
If you’d prefer a nice graphical interface, try Process Explorer, Sysinternal’s 268KB more feature-filled alternative to the built-in Windows Process Manager.