I've used NTpasswd on several clients' Vista machines and it worked
perfectly to reset the built-in Administrator's password to a blank. You
need to download the .iso and then burn it as an image. The download is
quite small as opposed to downloading an entire Linux Live CD distro,
which is probably just going to be using NTpasswd anyway. In XP, you need
third-party burning software to do this such as Nero or the free
CDBurnerXP Pro. I don't know if Vista's built-in burner software can do
images because I use Nero on Vista.
In any case, make your burned image CD and boot with it. Follow
directions. It is strongly suggested you READ the information at the
website first.
http://home.eunet.no/~pnordahl/ntpasswd/
Once you've gotten into the true Administrator account, go to the User
Accounts applet and make password changes as desired and WRITE THEM DOWN
AND PUT THE PAPER SOMEWHERE YOU WILL NOT LOSE IT.