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.