System Defender is a malware that fakes itself as a security tool. As soon as your PC is infected it starts generating warnings that appear like genuine system generated warnings. It keeps asking you purchase updates, dont click on anything or pay any money. There are several ways of removing System Defender. But the easiest is to use Windows System Restore to roll back your PC to a date when everything was fine.
Windows System Restore automatically creates restore points whenever a windows update is applied or when you install something new. You can use these restore points to go back to a good state whenever you suspect virus or malware infections.
Eventhough you can launch Windows System Restore from the desktop by going to
Control Panel → System → System Protection → System Restore
you may not be able to access control panel.
The steps to follow if you are not able to access Windows System Protection tools from your desktop is:
Step1) Restart your PC.
Step 2) At your login screen click the power icon which gives you options: Sleep, Shut down, Restart.
Hold down shift key and click restart.
On restart you will be presented with the recovery screen with the following options : Use a Device, Troubleshoot or Turn off.
Step 3) Choose Troubleshoot and select Advanced Options and then System Restore.
Step 4) You will be presented a choice of restore points. Choose one that you know to be safe.
System Restore only affects system files and programs, all your documents, photos and other media are unaffected. If this doesn't remove the malware or you dont have any good restore points, you may need to use antimalware software.
The above method is not specific to System Defender infection, System Restore is a good tool to remove most malware and virus infections.