Well, I don't know what is the most effective anti-virus program in the world.
But I have found this program consistently effective in getting rids of resource hogs, unwanted prorams, unwanted pop-ups, unsolicited request for subscriptions and non-functional deathware that generally screws up your computer. I recommend every Windows User to run it at least once, and it is free:
Running this tool has speeded up, reduced annoying pop-ups, improved functionality and stability and removed badly/non-functioning software from every computer I have used it on. Honestly.
Now make sure Windows Defender or Microsoft Security Essentials (which are also free) are turned on as they work, don't screw up your computer and won't hound you to the edge of insanity.
Why have I posted this? Because a McAfee product I installed over two years ago - (despite protesting that I didn't think it would work properly), that could not be made to work despite hours of their tech support, and that I thought I had fully removed - started screwing up my Windows security updates a few days ago, out of the blue. An anti-virus company's product that has been 'uninstalled' leaving a legacy that made my computer vulnerable to security threats.
Only the cynical would ponder if that is a very clever marketing strategy (Oh no! Windows Defender is broken, I'd better get McAfee!), complete incompetence or a unique but essential requirement of their software that doesn't apply to other company's products - you decide. But it is strange that McAfee is the only program that needs a separate stand-alone program to uninstall it completely when that could and should be built into the uninstaller.
Hopefully it is now dead and gone forever.