I'm assuming that it is automatically logging in when you start up? if this is the case there's a couple of things you can do.
1. go to windows control panel / user accounts and you can change your login there.
2. you can go to start shut down logoff than you can login with your original a count this is assuming you remember the username and password.
Also in your main drive where windows is installed you can see what accounts you have
example ( c:\C:\Documents and Settings ) We'll show you a list of users this is where your profile information is stored including your desktop and my documents for those users.