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If you are running on a Mac then I would just suggest letting Mac OS X take care of it self, no extra software needed.

If your on a Windows machine then I would suggest Kaspersky.

jon
 
If you're talking about windows then I suggest Avast, its free, just after the first 2 months you have to register on their website, and then re-register every 12 months. It has lots of features and options, I've been using it for about 6 months now. Also, if you want a firewall I suggest Sygate, lots of option with it, etc...
 
AVG Free Edition is another really good one. I've found it actually pics up viruses Norton didn't...
 
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