I use the free version of Avast! and am very happy with it. It seems to be very effective. I had Microsoft Security Essentials (I think that was the name of it--also a free one) briefly; it is a good program too but a little less compatible with my dial up internet--it was always trying to update as soon as the computer fired up, and then it would shut itself down because the computer wasn't logged onto the 'net yet--it doesn't take long for my computer to get connected, but the update was just a little too quick. Sometimes it would try again & work, other times I had to do a manual update. My tech said he had other dial up customers with the same issue, so he switched it to Avast for me & that works much better.