Windows Virtual PC is a virtualization module available for Windows 7 and enable users to install different versions of Windows on the same computer.
The program can be installed only on a few versions of Windows and, as guests, supports only a limited number of versions (Windows 7 and prior).
You can choose a custom amount of RAM memory and HDD space for each guest. Also, you can activate the Undo Disk option to backup each virtual machine in different moments.
The program is not very easy to use the first time, but once you will setup the first virtual machine, you will discover useful options and features to customize it. It can be a good solution if you don't like Oracle's Free VirtualBox.