I used to use AVG but they got too heavy with adverts. Then switched to avast which worked ok. One warning is NEVER use any "performance" tools your antivirus ships with, especially not registry cleaners, those can make a real mess of your system. On windows I also used malwarebytes as a free scanner, no realtime protection but good for second opinion file and system scans. After a while with avast I had problems from microsoft themselves, unrelated to my antivirus, of them pushing the GWX.exe "upgrade to win 10" malware to me. I quit Windows, went to linux and haven't needed an antivirus since. Even on linux though keeping your browser up to date is very important, and further securing it with NoScript (firefox only but a really good security tool) to stop drive-by download exploits is extremely useful.
As for what is a GOOD free antivirus for windows today I don't know, which is best varies pretty rapidly and I've been away from windows a long time. Almost all of them bundle junkware (harmless but irritating) toolbars and browser extensions thesedays, but most of them don't install the tolbars if you go through the full options during install and untick the toolbars. Also untick "performance" and "speed enhancement" tools, don't allow a "registry cleaner" to be installed
Whatever your OS the virustotal website is also a useful resource, lets you upload and scan files. You have to download the file yourself first, but you then don't run it until you've checked it with virustotal to make sure it is clean.