XP install of SP1a hung

Installing SP1a on my home pc, files downloaded ok,
restore point taken, files backed up, but then a hang
occurred while installing the new files. Now when I reboot
I blue screen after 10 seconds.

If I try to reboot from the recovery CD I am prompted for
the Administrator password - which I don't have - as this
is my home pc none of the user accounts use passwords, and
I can't find a record of the Administrator password.

1) Is there a way I can reboot from the CD and get past
the prompt for the Administrator password:

2) Assuming I can do that, what actions should I then take
to restore my XP installation to it's pre-SP1a state?

Thanks

XP install of SP1a hung