Why pay for a Anti-Malware solution if you can get one of the best solutions in the world for free – go and download it! It is there: http://www.microsoft.com/security_essentials/
And now, the disclaimer: It runs only on genuine Windows!
Have fun, enjoy. I am running it since quite a while with my friends and families and they all love it as they do not see and feel it at all – unless something bad happens. It is great!
I have been using it myself and absolutely loved it. It's one of the best Anti-Malware solution available in market.
After doing some stress testing, I am quite postive and can rely on this product.
Good work by Microsoft, I would say they striked back in wonderful way after One Care.