im not sure with vista, but at least with XP if you hit F8 and boot to safe mode you can access the ADMIN account if it hasnt been disabled or passworded as well, and you can reset passwords from there. a *LOT* of people's PC are subject to this lol
I've had to use that trick a couple times myself to gain access lol
but like i said, not sure if Vista would let you go in that way, and also heavily depends on her still having an open admin account......