Disaster alert: password not recognized after update

I run XP Home on a Compaq laptop. Sun 13Feb05 I rebooted as directed by
the update installer. I could not log back in: password not recognized.
(I did not change the password; I did not forget the password, I have
to type it every time I turn on the computer; I did not have CAPS on,
or NUMLOCK, key presses resulted in correct characters when typed
unmasked.)

Before the Windows update I had to do a MSN Messenger update, otherwise
wouldn't log in the Messenger service.

I did not install anything else for a very long time. I did not do
anything else unusual I'm aware of.

I ended up blowing the drive; luckily I backed up data a few days
earlier.