Unknown Publisher

I just upgraded a clients Terminal Server from 2000 to 2003. All of a sudden, when users login and try to run their primary application software which is located on a different server, they get a warning message that the publisher could not be verified; unknown publisher. This was never an issue with TS on Server 2000. How do I get around this; can I get around this?

I see all of these posts about the "Attachment Manager" in Group policy, but I see no such entry in the GP editor.

Thanks much for any help on this.