This FAQ tells you how to configure your host PC to give you network access to your virtual PCs by creating a virtual private LAN. It is mostly taken from an article by Aaron Zupancic plus a few other sites. I have seen this question asked several times so I thought I'd bundle it up into a FAQ. Note: this FAQ applies to Virtual PC - it's probably unnecessary in Virtual Server!
First, add the Local Loopback Adapter. This is accomplished (on WinXP) by:
opening the 'Add Hardware Wizard' (from the Control Panel)
indicating that 'Yes, I have already connected the hardware'
selecting 'Add a new hardware device' at the bottom of the list
selecting 'Install the hardware that I manually select from a list'
selecting 'Network adapters'
selecting 'Microsoft' and 'Microsoft Loopback Adapter'
Once it's been added, you configure the private network via 'Network Connections' in the Control Panel. I typically rename my network to 'Local Loopback Network' from 'Local Area Connection X'. I then assign it a unique, non-routable IP address (such as 22.214.171.124 or 192.168.99.1) - something that won't conflict with the various networks into which the host will be connected.
Then, open your VPC and manually assign an IP address to it on the same subnet (e.g. 192.168.99.2).
Edit the hosts file (found in %WINDIR%\System32\drivers\etc) to map the host name to its respective IP address.