Something I always find a little fiddly is networking virtual machines together in Virtual PC. I'm currently working on my Windows Server 2003 MCP (70-290) and to aid my learning, I'd like to have a domain controller running in a VM and also a VM XP client, and these are required to be networked. I'd also like them to have internet access. An easy way to configure your virtual machines to meet these requirements can be found in an excellent post at simon.says blog.
I hope you find it as useful as I did.