Cannot delete a file

I am trying to delete a file of our server but it just wont let me do it.
Shift + delete gives me “Access is denied, source file is in use” message.
Downloaded unlocker, It say there is no locking handle associated with that file but when I try to delete it, I get the message “Unable to delete file, would you like to do it at next reboot?”
I’ve checked on the server and their doesnt seem to be anyone accessing the file. Can anyone tell me a way of finding out who or what has the file locked so I can unlock and delete it before teh user who wants it deleted does my head in 🙂