can't access control panel after XP SP2 upgrade

The SP2 upgrade was supposed to be succesful, but it wasn't, so I would like
to uninstall the SP2 or go back to my previous configuration. My problem is
that the tool bar is gone and I can't access the control panel. The only
thing I can access is the task manager.
Another problem is that the computor is locked by me (blocking other users)
and I have to log in with a password to unlock it. I don't have any.
According to Microsofts home page, a temporary password is created during the
upgrade but should be removed when the upgrade has finished. If the upgrade
is not completed in the right way, the password could remain. In that case I
should be able to find it in the file setupact.log. I managed to find the
file, but there is no password in it.
Does anyone know how I can revert to my previous configuration (can't use
F8 as I have to log in with password) or uninstall the SP2?