Here is the scenario: installing an update to ActivCard (6.1) to a Windows XP laptop. After the required reboot, Windows XP brings up the default blue background, waits for quite a long time and then brings up the following dialog:
User Interface Failure
The Logon User Interface DLL acmsgina.dll failed to load.
Contact your system administrator to replace the DLL or restore the original DLL.
Serverfault seemed like a much better bet than trying to get a hold of our system administrators on a nice Friday afternoon....
EDIT: the machine cannot Safe Mode boot. It hangs at Mup.sys.