Hi, Since when you are facing this issue ?
Try running this :- http://download.microsoft.com/download/3/1/7/317254BC-6C9D-4532-827A-827041404428/MicrosoftFixit50195.msi
One file will be downloaded you just need to click on it and it will troubleshoot your problem and try to fix it.
It will open a window like this
Did this worked ?
Let me know if you issue still persist else you can close this request with your feedback. I will be waiting for your reply.