For a long while I used AVG free, which is very good, but one day something suspicious got through and I couldn't shift it. A friend who works in IT suggested I downloaded NOD32 from eset, as he said it was the best of the best. It is paid for, but quite reasonable and I haven't had any problems since that it hasn't picked up and eliminated immediately. My wife still uses AVG free, and that seems to work for her too. My friend in IT said that AVG free is better than many of the paid-for ones, but NOD32 is even better.
I also use Spybot to get rid of spyware too. I am running Windows 7, and have no plans to change.