Excel Files Stay Open

I have a user, that after upgrading him to Windows XP and Office XP, he has trouble with Excel Files on a network share that stay open after he closes them. This is evident because when he tries to open the file that he may have worked on the previus day, he gets a message that the file cannot be accessed. I go into Computer Mangement on the server, and look at open files, and see that the file is still open from the previous day. If I close the file, he can then open it with Excel.

At firstthis type of behavior was very frequent, and I disabled virus scanning for .xls files, and that helped. But it still happens for particular Excel files.

The network is W2K Native. The user has latest service packs for Windows and Office.

Any clues?