Add-in Already Open

A user has a macro stored in his start up folder. It's been working fine for
years until recently he started receiving an error message on start up which
says "A document with the name 'Reports.xla' is already open. You cannot
open two documents..." We can't find any other instance of this file on his
computer. If we uncheck the add-in in the Tools, Add-Ins dialog box and
close Excel it opens fine, but the next time Excel is closed and opened the
warning is back.

We're using Excel 2003 SP3. Has anybody experienced a similar problem or
have any ideas what might be causing it? Any suggestions would be
appreciated.

Thanks.

Erin