It's possible your computer might have damaged or corrupt system files. Run a System File Checker scan from an elevated command prompt (option two, this tutorial.) If problems are found run the scan 3 times rebooting in between each scan. SFC /SCANNOW Command - System File Checker
Microsoft also has this KB article that tells how to reset Windows Update components. How do I reset Windows Update components?
It's also possible that your machine might have been infected with a virus or trojan. It wouldn't hurt to run a full scan with your antimalware program of choice followed by a couple of on-demand scanners like Malwarebytes, Hitman Pro, Superantispyware, Comodo Cleaning Essentials, Trend Micro HouseCall, etc (all free.)