Hyper-V Program Manager
Microsoft supports having both Virtual PC and Virtual Server installed on the same computer at the same time. When a user does this there are two major 'shared components' between the two products. The virtual machine monitor (VMM.SYS) which is responsible for the basic creation and management of virtual machines, and the virtual machine network services driver (VMNetSrv) which is responsible for providing network connectivity for virtual machines.
One of the interesting side effects of having these shared components is that from time to time one of the components will be updated in one product but not the other. When this happens it is sometimes possible (though not always so) to reap the benefits in both products by having them installed on the same system.
A current example of this is that the Virtual Server 2005 Service Pack 1 beta contains a number of performance improvements - and many of these performance improvements can also be had under Virtual PC 2004 simply by install Virtual Server Service Pack 1 beta on the same system.