Once again Ben Armstrong, Program Manager of Virtual Machine Team at Microsoft, provides a veru useful tip for Virtual PC customers:
In Connectix Virtual PC 5.0 we moved to using a new BIOS and also moved from the 430TX chipset to the 440BX chipset.
One problem that this caused was that Windows virtual machines created on earlier versions of Virtual PC would not boot because they could not find the chipset they were looking for. The solution we came up with was simple. When booting a virtual machine would check the version information on the virtual hard disk, if the virtual hard disk was created with an earlier version of Virtual PC we would change the identifier on the motherboard chipset to the 430TX identifier (even though we were still emulating the 440BX chipset).
It turned out that this work just fine.