Choose the Shared Option in network adapters on the Virtual PC settings.

When you are on the virtual machine, you may need to go to the support tab of the “Local Area Connection Settings” and choose to “Repair”.

ipconfig /release and /renew are useful…

UPDATE FOR WINDOWS 7

I had to change the Settings on my Windows 2003 guest VM from Shared Networking to the network of my local wired connection.

Then, I had to change the Windows 2003 Local Area Connection for TCP/IP to automatically obtain DNS. This post details another approach that may help as well: http://www.tipandtrick.net/2008/fix-virtual-pc-2007-shared-networking-na…