Virtual PC 2007 was released on 20th February 2007 as a free download: http://www.microsoft.com/windows/products/winfamily/virtualpc/default.mspx.

The main feature is that it supports Windows Vista as both a host O/S and a guest O/S. It also supports PXE booting, 64bit Windows as a host and multi monitor systems where the VM can be full screen on one monitor.