I must agree with ohitsmonday about AVG. I too have used the free version for several years and never had an infection. With the recent purchase of a new system I also bought the professional version of AVG. It cost less than $35 (great with the current exchange rate) for a single PC license that is valid for two years. The free version uses the same detection files, but there are added facilities that I wanted, the main one being the auto-update as soon you go on line. The free version will only update if you're online at a certain time, and I don't leave my PC on and connected.
I run AVG Pro, Ad-Aware SE Pro (with AdWatch) and SpywareBlaster.
As an aside, a colleague was running the latest version of Norton with the latest update, but was still having trouble with what he thought was "just three trojans." I recommended AVG which he then installed, and told me it had found 132 trojans that Norton had been 'missing'.
Good luck and safe surfing.
A great Xmas and New Year to you all.
WP.