You would have to get the main user name (the one with administrator rights) and password and log in. Then once logged in go to Start, then hit the RUN icon and in the field type control userpasswords2. A screen will open up and uncheck mark the box that says users must enter user name and password, then hit apply. It may prompt you to enter the password on the next screen, do so and hit apply, ok, then thats it, you will never have to do it again.
Now if you dont have a way of getting the username and password, then you go here:
http://www.loginrecovery.com/, it will help you to recover the password. If it doesnt help, read through this article, has a few more places to try:
http://www.petri.co.il/forgot_admini...r_password.htm