I wouldn't ditch the anti virus software, it's very useful. I have noticed on a few of the comments that recommend AVG. AVG is good, but there are a few problems attached to it. You have to update the program constantly, they fail to automatically up date. Which can be dangerous because if you leave your computer a while it can be less protected.
Also - It lacks to communicate with Windows Vista and the last few updates e.g version 8 has seriously failed.
I use Kaspersky Lab. They have antivirus & internet security software and it has been extremely useful for me. It is 40 pounds but its really worth it.
Just stay away from the torrents!