Windows 7 was the first version that Microsoft was allowed (by the US/EU antitrust decisions) to have Defender installed out of the box in Windows. They were trying to do that as far back as XP and "Security Essentials" (the name before Defender) was a free and easy download on XP and Vista, but without the protection racket style marketing it was easy to miss and not everyone realized that there was a nice, quiet Windows anti-virus tool from Microsoft just a click away. I still think bundled anti-virus was one of the stupider things the antitrust decisions blocked and it gave the protection rackets a few more years to solidify power that they didn't need.

Actually an antivirus will only help if you already have malware. (Or if it scans downloads/websites) but it will, most of the time, not prevent new malware.