Safe mode starts Windows with a limited set of files and drivers. Startup programs don’t run in safe mode, and only the basic drivers needed to start Windows are installed
Safe mode is useful for troubleshooting problems with programs and drivers that might not start correctly or that might prevent Windows from starting correctly. If a problem doesn’t reappear when you start in safe mode, you can eliminate the default settings and basic device drivers as possible causes. If a recently installed program, device, or driver prevents Windows from running correctly, you can start your computer in safe mode and then remove the program that’s causing the problem
To enter safe mode of windows 7:
If your computer has a single operating system installed, press and hold the F8 key as your computer restarts. You need to press F8 before the Windows logo appears. If the Windows logo appears, you’ll need to try again by waiting until the Windows logon prompt appears, and then shutting down and restarting your computer.
If your computer has more than one operating system, use the arrow keys to highlight the operating system you want to start in safe mode, and then press F8.