Now that Windows Defender ATP is available for another Windows client such as Windows 7 and Windows 8.1, we can to defend malware and stop breaches from the same dashboard.
In Windows 7 SP1 and Windows 8.1 users use Windows Defender, Microsoft Security Essentials, Microsoft Forefront Protection or any of the other third-party anti-virus solutions.
The onboard older legacy operating systems with Windows Defender to ATP that currently available:

• Windows 7 SP1 Enterprise
• Windows 7 SP1 Pro
• Windows 8.1 Pro
• Windows 8.1 Enterprise

Its always recommended to use the latest Windows client versions, but there are scenarios where you need the advanced detection and response capabilities and of ATP and it’s not possible to upgrade the machines.
Also, the difference between Windows 10 and the older versions is that is not built-in and you have to install a Microsoft Monitoring agent which will connect to your workspace and report the sensor data.

Install and Configure Windows Defender ATP for Legacy Client

• Go to Windows Security Portal (WDATP)
• Download Windows Defender ATP client
  • A 64-bit agent is available here https://go.microsoft.com/fwlink/?LinkId=828603
  • A 32-bit agent is available here:https://go.microsoft.com/fwlink/?LinkId=828604
• Make sure you’ve got a direct connection to the following services via proxy, firewall, and OMS gateway

• Collect your workspace ID and Key

• Run installation with the following command:

setup.exe /qn NOAPM=1 ADD_OPINSIGHTS_WORKSPACE=1 OPINSIGHTS_WORKSPACE_AZURE_CLOUD_TYPE=0 OPINSIGHTS_WORKSPACE_ID= OPINSIGHTS_WORKSPACE_KEY= AcceptEndUserLicenseAgreement=1
In conclusion
The installation process for legacy Windows client is simple, but remember that you need to need the relevant requirements such as open the HTTPS services and configure the correct workspace id and key.