Windows Virtual PC (successor to Microsoft Virtual PC 2007, Microsoft Virtual PC 2004, and Connectix Virtual PC) is a program that allows an emulated Operating System environment to be run through an existing Operating System. The installed OS is referred to as the 'Host,' and the Virtual OS is referred to as the 'Guest.' Virtual PC is a useful program for attaining Legacy support for applications that do not run natively on the Host Operating System. In Microsoft's latest OS, Windows 7, Virtual PC is integrated into the Host OS, allowing Legacy programs of the Guest OS to run seamlessly. This is called 'Windows XP Mode.' More information is available at the following locations -
Download Windows Virtual PC (Windows XP SP3, Windows Vista, and Windows 7 Users)
Download Virtual PC 2007 (For Legacy OS support)
Windows Server 2008 and Microsoft Virtualization blog (Authored by a Microsoft Most Valuable Professional (MVP))
The following website contains helpful information for Virtual PC 2007 and Virtual PC 2004 users; advanced Windows Virtual PC users will find useful information as well.