I am using shared networking in my Longhorn VPC and I could browse the web and connect to the network. So to make working on the network a bit easier (and to try some more features) I thought it may make sense to add the Longhorn Virtual PC on my box to our domain. Setting this up worked.
Now, if I boot up Longhorn and my box is connected to the network, I get to see "Press Ctrl-Alt-Del to begin." and right after that, the VPC reboots. If I am quick enough to get to the logon screen, Longhorn reboots right after accepting the password. If I disconnect my machine from the network I can log into local LH accounts just fine.