Seems like a small problem with XP Mode and build 7137.
Switching the virtual machine from Full screen to Windowed mode (or the reverse) you are always prompted for a user and password. (Really irritating).
Then it gets the password hosed up so you get "Administrator account disabled" so you are really up that well known street without the paddle.
The only solution appears to totally "Wipe" the XP machine and re-install from scratch. (The XP machine -- not the Windows 7 Host system of course).
I did a Clean windows install (but kept my old Windows XP virtual machine) which *might* be the problem.