^ So the Genie is finally out of the closet eh ? You watch P**p P**n, I had no idea
In that case, you're obligated to use a full fledge security solution to defend you
Psst... Lemme' know if you wanna know about the good stuff
And to answer your question, the problem with both paid & free alternatives is this:
Once you get infected, there's no going back. Not a single AV is capable enough to entirely rid the system of the infections. The only permanent solution is a complete format, which is not very productive at all.
As for the friend's plugging their USBs in the PCs, how about you dual-boot Linux/Windows. Its what I do. Windows viruses don't work on Linux, and I've seldomly used a virus scanner once in a while just to clean out a nasty infected USB drive. You can copy your stuff without fear of getting infected. And since you now have a pretty good PC, you must try Ubuntu or Linuxmint in 64-BIT flavor. They're significantly faster and snappier than both Windows 7 & 8.