[AccessD] Access 2003 - Unknown Publisher?

I just installed a system at a new client, and I'm getting the Unknown
Publisher window each time it is opened.  I can push the 'Go Ahead' button
and the app works fine.  Macro Security is set to low.  I didn't expect to
see this because I have another customer also using Access 2003 where this
message does not appear.  And, it doesn't appear on my PC either.

Is there an easy way to turn this off?

Thanks!

Dan Waters