password problems?

hi. i'm new to this so please forgive me if i'm doing it wrong ...
i'm trying to help my mom. her pc is running vista and it loads to the password screen and then every password entered is wrong. she took it to office max and they told her it was a virus and they could do nothing. i have run avira rescue disc with the setting rename file if repair is not possible. again, i don't really know what i'm doing, but then i restart and i'm back to the password screen ... with no luck. please help :cry: