XP locking files in excel and word

In XP Pro, if you open a file in excel or word off the file server and then close it, and then try to open it again later, it says that the file has been locked by (user) do you want to be notified, or open as a read only. This happens when anyone with XP opens a document, closes it and then anyone trying to open later on gets this message.

You have to restart the computer that locked it to release the file it thinks is being locked. I have been told that it has something to do with caching?
Anyone out there had this trouble or have any ideas?
Thanks
Jerrett