Okay, so for some reason you don't have the password to your computer. Hmmm, why would that be? Maybe, you got real drunk one night and changed the password, or you're and distrustful spouse/partner that wants to see what his/her significant has on their computer. What ever the reason, there a few things you can do.
If you have your start-up disk or know the password to one of your admin accounts then you are good to go. Visit this site. They have the steps to changing your password.