Having strugled a great deal with Vista, "Add Printer dialog" and UAC have given my the grey hairs (if I had any).
Today I found the solution to Vista not wanting to install a network printer ("The specified print monitor is unknown").
Great errormessage, Microsoft! Who knows what ever that means? I have disabled UAC on Vista as it is just
pretty annoying to say the least. I consider myself an experienced IT-professional, hence I can detect when
to be aware of what I choose to install.
So when having disabled UAC and wanting to install a printer - you are in for trouble. It is simply not possible in the current build of Vista.
To resolve - open the registry "HKLM\Software\Microsoft\Windows\CurrentVersion\Policies\System" and set "EnableLUA" to a value of ONE "1" - reboot - and the printer adds just fine.
The question I'm sure you're asking yourself... "Do I have to leave UAC enabled???" - NO. You can easily disable it again after install and still be happy printing.