windows password

[knighty]

i have just bought a cheap second hand pc for my mum, when it logs on it asks for a password, the old owner told me he had deleted the password and it should not ask for one......but it does, i have tried no password, admin etc etc but it keeps asking me to ensure username and passwrd are correct HELP
i wondered if you pressed one of the F keys could it take you to a set up area to change password or better still reformat as i would liek the pc clean ?
thanks for your help guys