yess .. that ever happened to me mate, and i was so scared about that . becoz i can enter my own computer ( i forgot my password ). i try to reboot by CD and repair it and it still ask the password.
have u try to install it by reboot from cd and make a refresh installation ?
and when it ask you to format the NTFS or Fat32 .. just choose "leave the curent " and it will reainstall ur OS.
i know it is not the solution .. becoz at that time i don't know the sollution either. but let me give u an advice ( from a newbie ) .. always make more than one admin account on your system . i did that till now .. so if u forget you still have the other one.

i could be wrong but i've trid mate.