To start your virtual machines when you recieve the error "<machine> could not be restored because of either host processor type mismatch or lack of hardware-assisted virtualization support in the system." you will need to remove the hybernation file.
1) Open Windows Explorer and browse to "C:\Users\<username>\AppData\Local\Microsoft\Windows Virtual PC\Virtual Machines" replaceing C: and <username> as required.
2) You should now see the .vsv files for the VM with the starting problem.
3) Remove (or rename) the vsv file for that VM.
4) You should now be able to start your VM normally.