Files remain open even after logoff

I have a Win2K Small Business Server running SP3.

I am having a problem whereby when my users open files ( mostly Excel spreadsheets, but it happens with other data files also), then finish and exit the program properly ( some even log off the domain), the files remain in an Open state. I can see the status as Open when viewed using the Computer Management applet. The users will get an error saying the file is already open and they can only open a “read only” copy.
any ideas would be greatly appreciated.