PC affected from a power outage

I have a Dell PC with windows XP that was on and a power outage happened. The computer turns on and goes to the log-on screen that you can type a password to open it up to the Full Windows, but now the password won't work. It's like it changed. I'm able to get to the F2 or F12 as soon as the PC boots up, though. I have no restore CD as this was a hand-me down PC. So I'm not sure what I can do.