Viruses and Spyware are a real problem if you run windows, over the years I have tried all the options, pros and cons. I used to use Norton Antivirus, which I have always been happy with the protection but not the price, like $70 a year, which is pretty steep, personally any price on software is to steep, but there are many free alternatives...

Free Antivirus Programs:
 

Avast

This is my preferred Anti-virus application these days, although it is not Open Source it does auto protect, also has a premium version but the free one works great
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AVG

Another free one, lacks auto protect features in the free version.
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Clamwin

And another free one, lacks auto protect features. but is open source ;)
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Free Anti-Spyware Programs

Malwarebytes

the best free spyware protection available for windows these days..
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