First things first, remove Norton.
There are chocolate fireguards that are more useful than Norton is. It is quite easily the worlds worst anti-virus/internet security. The amount of computers we get at work that are fixed by removing Norton is unbelievable!!
Other than that, what people have said above ^
AVG Free edition is pretty good for a free AV. But normally you get what you pay for. Personally Kaspersky Internet Security is one of the best I've seen. I only use the Anti-Virus edition as I already have a firewall. 1 downside to the Internet Security is the firewall wants permission for everything, but it's no biggy really.
So to summerise, to stop getting viruses:
A decent Anti-virus and firewall (avoid Norton, Macafee etc.)
Use Firefox or Opera (avoid Internet Explorer)
Careful browsing (avoid the pr0n websites)
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