There was a System component called Microsoft Defender that made all other AV obsolete.
Obviously, this caused a lot of European AntiVirus vendors and Intrusion Tool vendors to get upset so there was a court case to prevent Microsoft from bundling Defender with Windows for corporate customers.
Microsoft is arguing that if it wasn’t for the Court Case artificially opening the market to incompetent vendors, the problem wouldn’t have occurred.
Windows has had some major security flaws over the years but ever since Vista, (and before that XPSP2), they have made a concerted effort to fix them. This has caused quite a few compatibility issues for programs that (ab)used these security flaws due to lazy or malicious programming.