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That you use another AV does not mean that Windows Defender is turned off (it did in previous versions). If your Virus & Threat protection looks like the image then it is turned on. Click on the arrow beside "Windows Defender Antivirus Options" there you can turn off Windows Defender. Or you can click Virus and Protection settings. There you can turn off just the folder protection.

Also check If Avast has an option to protect folders it may be called Ransom Ware Protection.

Notice that the above directions are only valid for a fully updated Windows 10. Before The Fall Creators update it did not have this.