VPC 2004 Tip #9: Changing the default location the Virtual PC uses for virtual machines

A regular question I hear is "How can I stop Virtual PC from storing all of my virtual machines in the 'My Virtual Machines' folder in 'My Documents'?". The answer is simple - but obscure. To change this you need to create a system environment variable called 'MYVIRTUALMACHINES' with a value of the path that you want Virtual PC to use for the default location for storing virtual machines.

Fonte: http://blogs.msdn.com/virtual_pc_guy/archive/2005/04/13/407796.aspx