Search google for password reset in XP or Vista. I do this for people all the time. Basically you'll have to find a downloadable .iso image that you'll burn to a cd. It makes a bootable disk that loads a unix based OS and boots into a script that will walk you through the reset.
I've had better luck setting to blank than typing in a new password.
NOTE: The times I've done this with VISTA afterwards I had to boot off of the VISTA disk into recovery console and run a "chkdsk /r" (was getting BSoD), but having said that....it worked and we were able to take control of the machines.
Good luck, and if you can't find an image pm me and I'll make an ISO for you.