How can I enable Windows Aero in a Microsoft Virtual PC 2007 instance?
The Virtual PC product emulates an S3 graphics card, which can't run Aero. However, you can still use the Aero interface by using RDP to initiate a Remote Desktop connection to your Vista Virtual PC from a Vista PC that's running Aero. Once connected, change to the Aero scheme and your Remote Desktop session will run Aero.
—John Savill