Assuming you've got networking working on it properly, you just boot the VM in VirtualPC and domain join it as per usual.
I'm not sure whether it's officially a "supported scenario" or not though, as various domain policies may well break some of the XP Mode functionality. I think that MED-V was the preferred route for scenarios that needed domain-joined XP VMs (it's been a while since I looked into it).
I'm sorry I haven't gotten back to you earlier. I was very busy standing up a new web server, migrating all of the web sites to it, etc. I'm only now checking my RSS feeds.
MED-V? What's that? Should I stop work on the Virtual-PC, delete it, and start all over with MED-V? Being able to join our domain is essential, so I've got to get this right.