It is important to know if you downloaded a program from someone.
If you did then they could have given you a Remote Access Trojan. And if it was a new one, or the only one of its kind, then no AV will pick it up.
Hopefully you actually weren't *hacked* and someone fished your account, or acquired your info using something other than a Trojan.
When they have your username/password they can log in to your account on the uo website, from there they can get your email, adress, last 4 digits of CC, etc...
Hopefully your email and UO passwords are different, and hopefully they didnt give you a RAT with a keylogger, because then your SOL.
If I were you, I would do a system restore. Then change ALL YOUR INFO at ultima-registration. Don't forget about changing PWs on email as well.
AV will not pick up all viruses, its a fact, so don't think your invulnerable there.
Knowing how they got your login/pw is important... If you got hacked, then it is a little more serious then a random fish.