Process Explorer

Do you want to see if your file or program is open elsewhere ? You can use this small tool for checking if your file handle in other files and more things.

If you are facing access is denied error in your programs . This is a great tool

Download : http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx

NOTE : If you are facing access is denied error on vista/7 it could be be because of windows resource protection feature of microsoft windows vista and windows 7
http://en.wikipedia.org/wiki/Windows_Resource_Protection