Excel is closing workbooks automatically

Hi

I have a user that uses excel quite heavily. Recently his system has developed a quirk where any open Excel workbooks automatically disappear from within excel. He is left with excel open and no workbooks visible. This seems to happen if none of the workbooks have been worked on for a few minutes. Does anyone have any ideas on how i can remedy this.

I have tried:
Reinstalling Excel
Repairing Office
Recreating his user profile
Getting him to work on a different machine

None of the above have fixed the problem. Any help would be greatly appreciated.

Harvinder