bypassing start-up password on my Dell vostro 230 with windows vista

I need help getting passed the start up password on a Dell vostro 230. I found it in a dumper behind Big-Lots. when I turn it on it boots up then go's to a black screen and ask for the password. I have tried pressing F2 and F12 nothing happens.can someone help me/ is there a back door i can use to reset the pc