|Can you get into Safe Mode? To get there you normally keep tapping the F8 button after you have put the power on - Safe Mode is shown on a menu.|
If you get there see if the Start button works. If so try sfc /scannow which checks and replaces system files - see More Information on the following page:
If that doesn't work see if you can run System Restore from Safe Mode. Choose a date before the problem came along. Mostly this is what folk have done to cure this symptom.
Always pop back and let us know the outcome - thanks