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If you have the money I would recommend that you go with the paid version. Its only about $54 a year, which isnt much. And I think that there are more features to the paid version (there usually is). But the free version is good too, its what I have on my fiancee's computer.
Oh, and if you dont have already have
Malwarebytes Anti-Malware, (its a virus removal tool) then you need to get it, now. It works wonders to clean up a computer from infections. And it wont conflict with an anti-virus programs.
You can get Malwarebytes here: http://www.malwarebytes.org/
There is the free version, and once you download it you can choose to purchase the full version. I haven't bought the paid version, but my father did and he likes it (the paid version actively watches for suspicious items, kinda like a quiet secondary firewall). But there isn't much added to the paid version that you don't get with the free one.
Plus you can use Malwarebytes to delete locked system files. That tool is really handy when trying to remove stubborn programs leftovers.
Remember:
Even with an Anti-Virus program, you can still get viruses and other malicious items on your computer. The first Firewall/Anti-Virus is YOU! Being careful where you go and what you do online is the main way to prevent any problems.
Some basics are:
*Dont open
ANY suspicious emails
*Dont go to dangerous sites (porn, illegal DL sites, etc...)
*Keep your computer updated
*Dont click on pop-ups, no matter how interesting or cool they may look (unless you are ABSOLUTELY sure they are legitimate)
*Watch others when they use your computer (or dont let them if you know they aren't careful)
*Dont loan your flash-drive to anyone
*And if you suspect you have a virus, deal with it right away. Before it can become a major problem.