Ok, this falls under the category of really reading the manual to fully understand this. As I wrote earlier, Virtual PC 2004 is now freely available from Microsoft. This is a fantastic tool to spin up virtual computers to try out new operating systems, or, try things to operating systems you wouldn't normally want to do to a "real" machine. A sandbox is the truest sense of the word.
Before I continue, I have read that Windows Server 2003 is not officially supported on VPC 2004. Doesn't mean it doesn't work, its just not officially supported. And, you should have a licensed copy of Windows Server 2003 to play with. Enough said.
Installation of Windows Server 2003 is the same as with any VPC "guest" operating system, and is covered in ample other locations. Once your install gets done you'll want to install all your patches Windows Update, maybe even Sysprep the VPC image so you can recycle it (great article on that here).
So now you have this fully functional server, what's next? Get your host machine to talk to it of course! That's where my fun began.
You may think that because you were able to access the Internet its a snap to access local network resources. Not so. By default, VPC configures networking to a "Shared Networking" setting. This creates a DHCP server inside of the VPC host, and with DHCP being the default TCP/IP setting inside of Windows Server 2003 you are good to go. But the VPC DHCP server (say that 3 times fast) acts as a firewall to network resources; you can't access them from the guest operating system.
The answer is relatively simple (now that I RFTM). After installing VPC, you have a new service available in your network adapter settings; "Virtual Machine Network Services". Check this on the adapter that is used to connect you to your network (and presumably the Internet).
Return to the VPC console, and select Settings on the Windows Server 2003 guest. Under Network Settings you will see your physical network adapter listed. Select it.
Start up the guest OS and make the following TCP/IP changes.
I could not get DHCP to work with my network gateway, so I specified a network address like I would for any machine on my network. This also required me to manually enter DNS addresses (do not use your gateway address; it won't work). So, for example, if your host machine's IP address is 192.168.1.11, set the guest OS to 192.168.1.20. Same subnet, same gateway. Adjust your address values according to your network settings.
At this point you should be able to ping the host machine from the guest OS, and vice versa.