AVG is a bit crap though. Avast/BitDefender are the current top picks. BD for the smallest footprint, Avast for additional features like mail scanning and so on.
I use BD because I only want a resident scanner on tap. Eset/MB etc are manually used for a 2nd opinion. I've never had an infection and my Win7 install is a release day install... Still runs like new.
I think Panda cloud has Bitdefender Free beat on small footprint,as to AVG and Avast they have been fine in my testing on Win8.1 over the last few months or so.
I currently use Panda Cloud and Avast on my two 8.1 PCs.
Bitdefender Free version I avoid due to gaming issues (ME3 crashing etc)when you try to run games like ME3 via Origin launcher.
Bitdefender Free lasted about three hours on my PC before I had to remove it.
I also use Malwarebytes Free version for manual backup scans now and then.
I don't know why anybody would not use an AV with Windows,there are plenty that are free,small footprint etc and no popups,its just another layer of added security which can't be a bad thing.
Linux I can understand since I don't use an AV on my Linux distros.