have you tried system restore? If that's not an option or fails to work try running the system file checker: Right click cmd prompt and run as admin. Type: sfc /scannow then press enter.
Also just as a tip, I run Avast as my security app coupled with Malwarebytes as I found Defender seemed to be lacking. It let through at least 3 viruses and so I changed over to avast and have been happy ever since. Try running a scan with Malwarebytes:
Lastly you can return win 8 to it's 'factory settings' to coin a phrase. If you place the cursor in the top right of your screen as if to bring up your charms and then hit settings, then 'change pc settings' and then finally General: See screenshot: