Update MTA
1. Install latest MTA update: https://nightly.multitheftauto.com/?mtasa-1.6-latest
Remove virus:
1. Install Avast: https://www.avast.com/
2. From Avast GUI: Scan->Scan for viruses->Boot-timeScan
3. Restart PC and wait for Avast to complete the scan
Repair Windows:
1. Start CMD as administrator
2. Enter command: sfc /scannow
(More help about this step from here: http://www.thewindowsclub.com/how-to-ru ... ws-7-vista)
Final solution:
If all else fails, the only fix is to re-install Windows.