Quoting from Microsoft:
Microsoft Virtual PC 2004 Service Pack 1 (SP1) contains the latest software updates for Microsoft Virtual PC 2004 that improve the reliability, performance and manageability of Virtual PC. Updates have been made to improve the performance of Windows XP Service Pack 2 as a guest in Virtual PC, to improve the manageability of virtual machines when used with SMS 2003 Service Pack 1, and to provide support for running Windows Server 2003 Standard Edition as the host operating system.
Some of the fixes included with SP1 have been previously released as separate updates. This service pack combines them into one update.
You can download it here.