[LightDestory]

I would suggest you to try out a "modded" version of Windows 10, a sort of a lightweight Windows 10 without Metro Apps, telemetry disabled, some services removed that will surely work better on your old PC.
Of modded Windows 10 there are two type:

• Updateable: It will have Windows Update still working and will receive updates;
  
• No-updateable: Window update is removed;
  

No-updateable are the most stable because the monthly cumulative updates will likely make your system unstable or will fail to install anyway.

This way you will be able to use Windows Defender, that is natively integrated with Windows 10