Hacking a computer pword?

i found a windows xp next to a dumpster the other day, brought it home, cleaned it, and pluged it in and it worked! the only problem is i cant get past the log in screen. i need some sort of hack or backdoor code to get me past. i've. tryed youtube and all that and non of it has helped, this place is my last hope. its NOT the Home Edition Windows XP, its the Professional Edition Windows XP. please ask questions if needed i really need your help. i need a way to like reset the whole system and start fresh.