No Administrator a/c appears in Safe Mode

Please could someone help, I have searched other threads but no luck.

My daughter bought pc through Dell, with Vista pre-loaded. She literally "plugged in and played", obvoiusly following the standard promts as per setup as she went along. (I wasn't present at the time).

After using it for some time, she has now setup her account with a password.
Her account has administrative rights and is the only user a/c on the pc.

She has gone away to Uni and will not tell me the pw.
I want to utilize the pc for my son. ie create another user a/c for him.

I have started up in F8 Safe Mode to access the Build-in Adminstrator a/c
but no Administrator a/c appears at the welcome/login screen. (on my XP pc it does).
Only her icon & name appear.
The idea was to setup a new user account from Build-in Administrator login but how can I, when no Build-in Administrator icon appears.

I tried a password revealer Ophcrack from a free download.
But results came back:-
Administrator pw empty
Guest pw empty
Stephi pw NOT FOUND

Is there anyway I can login as build-in administrator?
Is there any software I can download that can allow me to do this?
Is there anyway I can create another user account without logging in to hers?

Appreciate some help, I'm not a computer geek. Please be clear on "administrators", I get confused between "user administrator" and Build-in administrator".
Many thanks, Rose