have to use guest account in windows vista, lost password, please help

im just after getting this laptop, a month back or so. and i tried to change the password at one point, but it didnt seem to work. so when i logged off i thought it hadnt changed. then when i went back on, neither the old or new password worked for me… i cant log into the main account, have to use a guest one

its windows vista, there is no disc with the password etc. and i have no way of getting the password.
so i have no idea what i can do, can someone please help?