Peter Provost says he used Handle from Sysinternals. Handle is a great little tool I wasn't aware of: it tells you which processes have a file open. It is particularly useful when you have a problem with a file beeing locked, but do not know what put the lock on it.
Sysinternals has tons of other useful little tools like this. I remember that their first tool I used was NTFSDOS to read NTFS partitions from DOS. I happened to play with their tricky Blue Screen Of Death screensaver as well.