Received a new CyberPowerPC last week with Win 8 installed. Gets the job done. I went with the idea that the Windows 8 claim that Defender does it all now so no outside Anti virus protection needed. NOT the case for me.
About 4 hours into playing with the new system I get and screen with FakeAV on it and it started its routine. Being
a new system there was no way this should be happening. i ran Defender again, and it saw nothing. I've used AVG a long time so installed a new version for it, and did full scan. It found it about 10 minutes later. Thats was enough proof for me that Defender isn't the last word yet and just run with AVG and Malwarebytes now. Been on all week and no more problems. I've used Avast in the past on my other systems with no complaint also. I prefer AVG but to each his own. Defender didn't do it for me, maybe you will have better luck.

Swampy Yankee