I have picked up the same problem, I didn't do the install just fixing the problems. A server running Windows 2000 AD etc and workstations running Microsoft XP O/S and also Office XP. The Excel files were originally created on Office 98, I think.
Open up an excel document and close it, the file remains open on the server side and the server retains a lock and doesn't want to release, we have tried all sorts of different opening and closing procedures and ultimately the server retains the file locks. Office XP is service packed upto SP2 I think?
If anyone has any further suggestions I would be interested.
Cheers