Logging in as adminstrator

I have a dimension 4500s with windows XP. Somehow I changed my log in from adminstrator account to a guest account. This will not allow me to use several features such as systems restore. I have used run and typed in "control userpasswords2" and it brings up a user account box with user name "adminstrator" and ask for a password. When I go to the control panel and open user accounts it just shows the guest account. How do I find out what the password is that I assume was orginially assigned by Dell