Panda Platinum Internet Security for me. It's not free... but then I'm one of those that believes you get what you pay for in a lot of cases. It updates definitions daily, has firewall protection and also spam / spyware protection. It takes up less resources than Norton, and I have found it offers better protection.
Toey recommended Panda to me a while back, and it has worked very well for me. Thanks Toey!
<font color=red> If you design software that is fool-proof, only a fool will want to use it. </font color=red>