jamesk. As The Toy Man has already explained, Windows Defender (which is free) comes included with Windows 8.
As I want to keep my SP2 as clean as possible (that is from unnecessary program installations) I refuse to install any third-party anti-virus solutions.
At the end of the day, the protection you receive from antivirus solutions differs from month to month. That is, one month Microsoft Defender may stop more common threats than AVG. The next month, however, AVG, or Avast!, or any other solution you can think of, might top the list.
What you have to appreciate is that new threats are developed every day. And, it's only by chance (I say chance because there's a large number of variable factors that come into whether or not a threat is discovered) that whoever is updating the databases comes across these threats to be able to improve their software.
All these companies have good and bad months.
In fact, I've stopped worrying about whether an antivirus solution will stop everything coming through (because, as I've explained above, you can never guarantee that) and instead have focused on how the solution will impact the performance of my every day use of my SP2.
Just make sure you have some protection in place and be careful what you click on.