When I build a new system or upgrade an OS, I usually keep my file structure the same and as such, the shortcuts for my VMs stay the same.  Since I usually have around 30-40 VMs installed in VPC, it's a pain to manually add them via the wizard or simply double-clicking the short-cuts and launching them.

In XP, Virtual PC stores the short-cuts it uses to populate the VPC List in

X:\Documents and Settings\%username%\Application Data\Microsoft\Virtual PC\Virtual Machines

In Vista, this location has changed to:

X:\Users\%username%\AppData\Roaming\Microsoft\Virtual PC\Virtual Machines

It took me a while to figure out where the new location was...actually, I create a temporary VM with a unique name and searched for it.  Once I found it and copied my shortcuts over, my VPC 2007 list is just as fully populated as it was last night before I upgraded my main system to Vista Ultimate x64.