How do I recover/reset the admin password of a Virtual PC instance
Solution 1:
The same way you would reset the passwords for a physical Windows machine. My preference is Offline NT Password and Registry Editor. It will let you change or reset any of the local passwords on the system.
Solution 2:
If all you're trying to do is recover the data then why not just open the .vhd disk file? No muss, no fuss. Plenty of tools out there can open these for you.
Solution 3:
You'll need a password recovery disk. Then change the drive used by the VPC to be the physical CD drive on the machine. Then choose to boot from the CD drive when the VPC boots up.
Solution 4:
Not a VPC problem so much as a Windows one... The following article from Daniel Petri will help with a lost/forgotten administrator password: http://www.petri.co.il/forgot_administrator_password.htm