Mhmm, what do you mean by "bad" ?
If he just created an other account from your account, what he probably did, log in it and simply delete his account from the preferences panel.
Unless he changed your own password or created an other administrator account, loged in it, and delete your account...
Now if it happened, I can just advice you to reset the password using the original OS X installation DVD, which means you own the mac, it's a "thief-security".
If the CD is bad, then go to the Apple Store and explain them, they'll reset your password.