ARGH!

My sister brought home a school laptop to print some stuff, and I was going to add it to the workgroup for it to access the network printer, but now when I log on it doesn't recognise the password for the administrator account?

Is there any legit way to put the computer back onto the domain and save my arse? I know what the administrator password is for when it was logging onto the domain... but apparently there different for workgroups?

HEEEEEEEEEEEEEELP.