Great question—let’s get straight to why you’re hitting this error:

• Set-MpPreference is tied to Windows Defender, which isn’t part of Windows 7
  This cmdlet is part of the built-in Defender PowerShell module, which Microsoft introduced starting with Windows 8 and Windows Server 2012. Windows 7’s default antivirus (if you have one installed) is Microsoft Security Essentials (MSE), not the full Windows Defender suite that includes these PowerShell tools. Even with MSE installed, it doesn’t include the Set-MpPreference cmdlet or the underlying module.

• PowerShell 5.1 doesn’t add missing OS-specific modules
  Upgrading to PowerShell 5.1 on Windows 7 updates the PowerShell engine itself, but it doesn’t magically add modules that are exclusive to newer Windows versions. The availability of Set-MpPreference depends entirely on your OS version and the antivirus software that ships with it—not just which PowerShell version you’re running.

• The threads you looked at don’t apply to Windows 7
  Those discussions about Add-MpPreference or Set-MpPreference -DisableRealtimeMonitoring failing are for systems where the Defender module does exist (Windows 8/10/11, Server 2012+). Their fixes won’t help here because the module isn’t even present on Windows 7 in the first place.

If you need to tweak antivirus settings on Windows 7, your best bet is to use MSE’s command-line tool MpCmdRun.exe or adjust settings through the MSE graphical interface—there’s no direct PowerShell cmdlet equivalent for Set-MpPreference on this operating system.

内容的提问来源于stack exchange，提问作者magikarp