Well, this is a shot in the dark, but it may well be worth updating to SP3. There's a fix to a signature problem in it. If the workbook was originally 2000 or 97 this could be the problem - modules
are being deleted and of course that would cause the message. See below link.

http://support.microsoft.com/kb/8360...23120121120120


Nadine wrote:
MS 2002, SP1

"Gareth" wrote:


If you've protected it then, it can't be modified programmatically either. Strange. I've seen Excel 97 (a known bug I believe) insert lines into a locked project's modules just by opening and closing
the workbook.

What version of Excel and SP are you and he using?

ben wrote:

If you don't know for sure then it's a good guess that it does not do so.
It's not a simple process to do. Have you tried installing the security
certificate on the users machine? They won't be able to write anything with
it, their machine will just recognize it as trusted.