lost admin account

Hi All,
I'm Trying to help out a friend who has lost all but a limited account.
He had an administrator account and two limited accounts for his kids. When he came back from vacation and booted up all that shows is one limited account. He is running XP Home and can make no changes not even a system restore. Can someone please let me know how he can get back in to be administrator?

Thanks in advance,

Dan

 

Many thanks TimW for a quick and reliable reply.

Thanks, :wave
Dan