XP and Office 97

I have 2 PC that are running XP and Office 97 on our network with W2K servers. From time to time this situation happens:

The user opens a Word document on the server. He makes some changes then closes it. He goes to open it again and it says it can only open in read-only mode. I have check the server for open files and it says it is open. I re-check the users PC but it isn’t open. I can either manual close the file or have them logoff then the file closes. I have had similar situation with Excel spreadsheets.

Does any one have any ideas why this might happen?

Thanks,
Bill Mace