Ultimately, the successor of Virtual PC 2004 is released. It's Virtual PC 2007. It runs on VISTA (Wow).
Download from http://www.microsoft.com/downloads/details.aspx?FamilyId=04D26402-3199-48A3-AFA2-2DC0B40A73B6&displaylang=en
An x64-based or an x86-based computer with a 400 MHz or faster (1 GHz recommended) processor with L2 cacheProcessor: AMD Athlon / Duron, Intel Celeron, Intel Pentium II, Intel Pentium III, Intel Pentium 4, Intel Core Duo, and Intel Core2 DuoRAM: Add the RAM requirement for the host operating system that you will be using to the requirement for the guest operating system that you will be using. If you will be using multiple guest operating systems simultaneously, total the requirements for all the guest operating systems that you need to run simultaneously. Available disk space: To determine the hard disk space required, add the requirement for each guest operating system that will be installed.Virtual PC 2007 runs on: Windows Vista™ Business; Windows Vista™ Enterprise; Windows Vista™ Ultimate; Windows Server 2003, Standard Edition; Windows Server 2003, Standard x64 Edition; Windows XP Professional; Windows XP Professional x64 Edition; or Windows XP Tablet PC Edition