If you have not successfully logged on after the problem first occurred, press the F8 key when the operating system selection page is displayed or before the computer starts Windows. Select the Last Known Good Configuration option,
If you still experience the problem, follow these steps to log on to the computer and to run Sc.exe.
1. Restart the computer.
2. Press the F8 key before the Windows logo page is displayed.
3. Press the F8 key to select Advanced Startup Options when the operating system selection page is displayed.
4. Use the arrow keys to select Safe Mode with Command Prompt, and then press ENTER