Shutdown GPO

My apologies if I am in the wrong place. We have a computer shutdown GPO
attached to an OU where a bunch of XP and VISTA workstations are. This GPO
works fine on XPs as they are shut down or restarted. The GPO does simple
things like setting DNS or WINS or other things are commanded via parameters.
On VISTA however, it only runs if the user manually runs the same binary.
When run via shutdown GPO on VISTA, the shutdown appears to freeze for ever.
We have to hard boot the system. We have figured out the problem is due to
interactive nature of the application. For example, it sets the DNS on VISTA
correctly, but when it displays a prompt to the user, the prompt is not
visible to the user and hence he is unable to acknowledge and respond to the
prompt. I figured it is due to the display message going to another desktop
perhaps? Under VISTA what is the desktop to use after the user has logged off
and the system is shutting down? Perhaps I have to send my messages to a
specific target desktop? Here is the VB code snippet:

------ some code ----
If DNS is successfully setintDNSServers = 0 Then
MsgBox "DNS successfully set.", vbSystemModal, "Shutdown-GPO" ' gets
stuck here forever
Else
MsgBox "Error setting DNS. Please contact an administrator
immediately.", vbSystemModal, "Shutdown-GPO" ' gets stuck here forever
End If
-------------

Any assistance will be appreciated.

Thanks