try this
while the computer is starting up, hit F8
select Safe Mode
log in as Administrator with blank password (hopefully your previous user didn't set any password here)
once windows starts up, go to control panel and create a new account for yourself
reboot

if the administrator account also has a password and you don't know what it is, and there's no way for you to contact the person who set the password, the only way is to look into password unlock tools or reformat the drive and install windows fresh