i would have to disagree with the recommendation of norton.
if you look at the threat detection rate norton is normally on the lower end compared with similarly priced and free solutions. also be aware that if your subscription runs out the av stops working... kapersky will still work but wont get updates. the only thing norton does is provide almost zero popups so it is often used by people who arent very computer literate.
If you want free antivirus then I personally suggest AVAST.It is very effective.I have used it for one year and my machine was never ever affected by any viruses,malwares,trojans...
If you want to buy some antivirus then you may go for KASPERSKY.I have previously used it and it has rock-solid firewall,anti virus features.But it is pretty costly.So better go for AVAST antivirus.AVAST is very light-weight and consume least resources.You will not feel it running behind the programs and giving you protection.
KASPERSKY is a bit resource consuming on the other hand.It slowed down my PC enormously during a full system scan.Also KASPERSKY take more time to completely scan your PC.
Now,I have installed windows 8 CP.So it has WINDOWS DEFENDER by default.I am using it.It's pretty cool.But I cannot say now about its effectiveness as I am using it for only a couple of days.