I use Windows Live OneCare. It's good, simply put.
Before I was wishing to dish out money for onecare, I was on windows firewall and AVG Free Edition.
A simple antivirus works for everyday joes, and if your behind a router/network, windows firewall will get the job done.
If your pirating and all that fun shit, your asking for trouble either way, protected or not.
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I'm that Josh kid with the cool avatar.