Is it Windows XP Home, Professional, or Media Center?
If it's any of the above except Home, there's this old trick to "hack" into it due to shitty Microsoft design. On the login screen after Windows starts up, hold CTRL + ALT and tap Delete twice. Type "Administrator" as username and hit enter and you're in. This, of course, doesn't work if the password was changed.
Last edited by WWONG; 09-18-2011 at 03:05 AM.