Strongly recommend using Windows Defender which comes natively w/ Windows 8 and is ON, by default. No real need to install a 3rd Party program or security software. It is, already, in the OS & saves having to install anything or something else, instead.
I think Defender has to improve a lot. It does not fare well against other AV products in tests. As Windows 8 becomes more widely used it's going to be a constant big target for hackers like Flash and Java.