Excel 2002 Office XP File in use error

We are on a Win2k Server network system. When trying to open spreadsheets we get FILE IN USE ****.xls is locked for editing by:.. then a choice to READ ONLY, NOTIFY OR CANCEL.

As no one has these spreadsheets open it is getting hard to try and locate the problem.

The data file is on the server and the network system is 1 server + 5 workstations.

This problem appears to be affecting Excel 2002 only.

Your help is appreciated.