Quick tip if you are intending to install Virtual PC on Windows 7 (i.e. use Windows 7 to host other OSes, not install Win7 as a guest OS), and are upgrading from Vista.

1. Uninstall your existing Virtual PC instance

2. Run the upgrade from Vista to Win 7

3. (Re)install Virtual PC 2007 SP1

If you don't, then you can hit a problem with virtual networking / network sharing. (Although uninstalling and reinstalling VPC while in Win7 also seems to work)