"Virtual PC 2004 Service Pack 1 (SP1) is a powerful software virtualization solution that allows you to run multiple PC-based operating systems simultaneously on one workstation, providing a safety net to maintain compatibility with legacy applications while you migrate to a new operating system. It also saves reconfiguration time, so your support, development, and training staff can work more efficiently," says Microsoft.
It runs on Windows 2000 Pro SP4, Windows XP Pro and Windows XP Tablet PC Edition. The download is 18.2MB (it took 2 minutes 41 seconds here).