Sounds like you have a virus that turned off your anti virus. thats about the only reason i can think of that you'd have this problem.
I've seen this a few times before.
Serves you right for using that piece of shit Nortons crap.
Get this one if you really want to do something about viruses, other then spread them.
http://www.kaspersky.com/
Made by Russian scumbag hax0rs.. so you know its going to do a good job.
