I used the HD2VHD (from Sys Internals) wit the HAL (Hardware Abstraction Layer) enabled.
However, when I try to run it, the Virtual PC guest doesn't have internet access. And when I called Microsoft to activate it they said the hardware had changed and I'd need to change the Activation Key (which I can't do b/c I don't have the original install CD).
I'm guessing this would happen even if I had internet access. Also, I've tried changing the Networking option within Virtual PC ( per this Answer ) and still don't have internet access.