If your computer will not start, you may be able to start it in Safe Mode. In Safe Mode, Windows uses default settings (VGA monitor, Microsoft mouse driver, no network connections, and the minimum device drivers required to start Windows).
For example, if your computer will not start after you install new software, you may be able to start it with minimal services in Safe Mode and then change your computer settings or remove the newly installed software that is causing the problem. You can reinstall the service pack or the entire operating system, if necessary.
1. Safe Mode
Starts using only basic files and drivers (mouse, except serial mice; monitor; keyboard; mass storage; base video; default system services; and no network connections). If your computer does not start successfully using Safe Mode, you may need to use the Recovery Console feature to repair your system.
2. Safe Mode with Networking
Starts using only basic files and drivers, plus network connections.
3. Safe Mode with Command Prompt
Starts using only basic files and drivers. After logging on, the command prompt is displayed instead of the Windows desktop, Start menu, and Taskbar.
4. Enable Boot Logging
Starts while logging all the drivers and services that were loaded (or not loaded) by the system to a file. This file is called ntbtlog.txt and it is located in the %systemroot% directory. Safe Mode, Safe Mode with Networking, and Safe Mode with Command Prompt add to the boot log a list of all the drivers and services that are loaded. The boot log is useful in determining the exact cause of system startup problems.
5. Enable VGA Mode
Starts the computer using currently installed video driver at the lowest resolution. The basic video driver is always used when you start in Safe Mode (either Safe Mode, Safe Mode with Networking, or Safe Mode with Command Prompt).
6. Last Known Good Configuration
Starts using the registry information and drivers that Windows saved at the last logon. Any driver settings or other systems setting changes made since the last successful logon will be lost. Use only in cases of incorrect configuration.
7. Debugging Mode
Starts while sending debugging information through a serial cable to another computer. The serial cable must be connected to the COM1 port with the baud rate set at 115,200.
If you are using, or have used, Remote Installation Services to install Windows on your computer, you may see additional options related to restoring or recovering your system using Remote Installation Services.
8. Disable Automatic Restart on System Failure
Specifies that the computer will not automatically restart whenever the system unexpectedly stops. Use this option to keep your system from restarting while you troubleshoot system failures. Starting with this option is the same as opening the System control panel, selecting the Advanced tab, clicking Settings under Startup and Recovery Settings and on the Startup and Recovery dialog box, clearing the Automatically restart check box under System Failure.
Important: Even if you disable the local administrator account, it will still be available when you use Safe Mode.
Thanks to: www.microsoft.com