I have this same problem. Every few days Windows defender is disabled, Bigfarm and another shortcut appear on Desktop, and a host of malwares appear when I scan with Malwarebytes/AdwCleaner. I scan & clean them, then the cycle happens again.
Please run the following steps:
Press on the Win + R keys, type in mmc.exe and press on Enter.
In the Console, click on the File menu, followed by Add/Remove Snap-in....
In the left list, select Certificates and click on Add to add it to the other side.
In the prompt, select My user account and click on Finish, then on Ok.
Browse under the Certificates - Current User till you reach Untrusted Certificates and click on it.
Please send me a scheenshot of the untrused