Virtual PC allows to create virtual machines with the same or different OS running on it. These virtual machines will exist and run in parallel with the host OS. Virtual PC software can be downloaded for free from Microsoft portal.
Virtual PC 2007 SP1 can be downloaded from the below location.
This can be installed on Windows XP and Vista operating systems.
For Windows 7, Virtual PC 2007 is not officially supported but you can still install and use it. Check the below article on MSDN blog for more information.
Download link for Virtual PC 2007.
Windows Virtual PC
On computers running Windows 7 and supporting hardware virtualization, we can install Windows virtual PC. Note that Virtual PC 2007 and Windows virtual PC are different technologies.
The above link allows you to download Windows virtual PC with or without XP Mode.