Again, you will need to burn Defender to a CD or write it to a flash stick using another computer. Read how in the link provided. First determine if you have Windows 7 32- or 64- bit by looking at Control Panel>System.
Also in Safe Mode install and run a full scan with Malwarebytes.
See if you can download and install it in Safe Mode with Networking. If not download it to a stick or CD and transfer it to problem PC in Safe mode to install and run it.