Try running the SFC, it's a good idea to run it 3X because it doesn't always get everything on the 1st or 2nd pass.
Also, run this tool to check if you might have picked up a rootkit.
If SFC could not fix something, then run the command again to see if it may be able to the next time. Sometimes it may take running the sfc /scannow
command 3 times restarting the PC after each time to completely fix everything that it's able to.
When running TDSSKiller, launch the program, click on the blue text "Change Parameters" & check the box marked "Detect TDLFS File system." Click OK & then run the scan.
This tutorial lists several options for enabling Defender:
Windows Defender - Turn On or Off in Windows 10 - Windows 10 Forums