April 2013 updates contains
http://support.microsoft.com/kb/2813170. It breaks Virtual PC Additions on Windows Server 2008 on system thatdoes not support hardware virtualization.The guest becomes very slow and takes 100% CPU (behaves as the VM Additions wouldn't be installed). The workaround is to uninstall the KB2813170from the guest OS. My setup is:
Host: AMD Athlon64 (no hw virtualization support), Windows XP SP3 x86, Virtual PC 2007 SP1 version 6.0.210.0
Guest: Windows Server 2008 Standard SP2 x86 (from MSDN), VM Additions version 13.823
Note that other Virtual PC guest OSes Windows Server 2003 R2 SP2 and Windows XP SP3 are not affected by the update.
Petr