Windows Defender Service Failing to Start

I've searched for so many hours for a solution to this problem but haven't found a working answer.

The errors I get are 0x800705b4 and 0x80070005

I recently deleted some old system files from an earlier Windows version (windows.old and another folder) and since doing so have noticed that Windows Defender isn't working and fails to start.

I've tried all of the basics, restarting the PC and removing any other anti-virus software (that I am aware of).

I've also tried to start it from services.msc and tried the sfc /scannow command in command prompt which neither have helped (failed to start service) and (no integrities found).

I've seen people recommend doing a System Restore, but I don't have an option for it nor do I want to.

What I would really love to do is re-install Windows Defender because I think this is due to the program using the old system files to run. Any tips on how to fix this or find a download for Windows Defender?

Thanks!