David Berlind, a ZDNet blogger, has some very useful details about the Virtual PC Express for Vista revealed last week:
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Virtual PC Express enables only a single VM. That is key difference between Virtual PC and Virtual PC Express. With the former you can have an unlimited number of VMs and with the latter you can only have one. Virtual PC Express [will be the version of Virtual PC that’s included] in Windows Vista Enterprise and Windows Vista Ultimate.
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For any additional VMs on the device, customers must purchase retail (otherwise known as shrink-wrap or FPP) copies of the operating system. For the example described above, the volume licensing customer would receive the right to install one copy in a virtual machine. The customer would then purchase 2 additional retail licenses to install in the 2 additional virtual machines (making 3 VMs total). These rights are associated with volume licensing of the operating system rather than with the Virtual PC SKU and so it makes no difference whether the customer is using Virtual PC Express or Virtual PC…
Read the whole article at source.