It seems that this problem isn't caused by a virus, but by a corrupted Windows Defender. Due to the fact that you can't reinstall this service, the only thing that you can do, is to use a working Windows and go to : C:\Program Files\Windows Defender. From there, create an archive with the Windows Defender folder, transfer it to your computer, and extract the files under the same location. After you did this, you should be able to access Windows Defender from Control Panel, and also download files from Internet.
