For years I have tried lots of different antivirus programs including AVG and Norton Antivirus. However I have found that AVG still lets through some virus' and allows them to embed themselves in your system.
Norton however it may be very accurate however it embeds itself deep into your computer system and takes over and is a pain in the backside to completely remove from your computer and slows your system down a lot.
I have been using another free antivirus known as Avast. I have never had any problems with it running on my Windows PC, and I have not noticed the system slow down by much at all and does not effect the performance speed of your system. Avast also has an active monitor which is continually scanning websites that you visit, if it thinks that a website is infected then it stops you from viewing the site and warns you that its infected before anything can be downloaded to your system. As well as a live website scanner to protect you from unwanted and malicious software it has an active email scanner too, so if you get spam email containing a virus it will prevent your system from being infected.