My administration rights were usurped!

Let me explain the situation. I have windows XP and originally only had a guest account and a personal, password protected user account.

Somehow without learning my password, my roomate made a NEW admin account, and somehow stripped my account to “unknown” account status. Now I can’t reach certain files, download or uninstall anything.

Does anyone know just how the heck this happened? And is there any way of regaining my administrator rights back without having to reformat?