I have a nMP with D700 graphics cards, basically the fastest thing Apple makes right now and for games it sucks. And don't say 'that's because it's got a workstation graphics card' They are basically the same cards, it's only the driver that changes and in bootcamp the are crossfire enabled, if you choose them to be configured that way.
I also have an old PC, i7 3770K with GTX 780, and at 1440p (i.e. the native resolution of the non-retina 27" iMac) it flies. Much quicker than the nMP does running the same OS (Windows 8.1) with the same games (Skyrim, Fallout New Vegas, Elite Dangerous). Perhaps the nMP will work better with other games, but so far the PC is tons better.
If playing games is your main reason for getting a computer then stick with the PC and understand why you got a virus. Probably because you downloaded some game hack from the internet? If you stay away from dodgy websites and use some discipline then Windows is a better platform for what you need.
It is because it's a "workstation graphics card" though the drivers for which are optimised for vastly different applications than gaming.