http://windows.microsoft.com/en-US/windows-vista/Advanced-startup-options-including-safe-mode

Advanced startup options (including safe mode)

The Advanced Boot Options menu lets you start Windows in advanced troubleshooting modes. You can access the menu by turning on your computer and pressing the F8 key before Windows starts.

Some options, such as safe mode, start Windows in a limited state, where only the bare essentials are started. If a problem does not reappear when you start in safe mode, you can eliminate the default settings and basic device drivers as possible causes. Other options start Windows with advanced features intended for use by system administrators and IT professionals. For more information, go to the Microsoft website for IT professionals.

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Repair your computer

Shows a list of system recovery tools you can use to repair startup problems, run diagnostics, or restore your system. This option is available only if the tools are installed on your computer's hard disk. If you have a Windows installation disc, the system recovery tools are located on the installation disc. For more information, see Startup Repair: frequently asked questions.

Safe mode

Starts Windows with a minimal set of drivers and services. For more information, see What is safe mode?

Safe mode with networking

Starts Windows in safe mode and includes the network drivers and services needed to access the Internet or other computers on your network.

Safe mode with command prompt

Starts Windows in safe mode with a command prompt window instead of the usual Windows interface. This option is intended for IT professionals and administrators.

Enable boot logging

Creates a file, ntbtlog.txt, that lists all the drivers that are installed during startup and that might be useful for advanced troubleshooting.

Enable low resolution video (640 × 480)

Starts Windows using your current video driver and using low resolution and refresh rate settings. You can use this mode to reset your display settings. For more information, see Change screen resolution and Correct monitor flicker (refresh rate).

Last Known Good Configuration (advanced)

Starts Windows with the last registry and driver configuration that worked successfully. For more information, see Using Last Known Good Configuration.

Directory services restore mode

Starts Windows domain controller running Active Directory so that the directory service can be restored. This option is intended for IT professionals and administrators.

Debugging mode

Starts Windows in an advanced troubleshooting mode intended for IT professionals and system administrators.

Disable automatic restart on system failure

Prevents Windows from automatically restarting if an error causes Windows to fail. Choose this option only if Windows is stuck in a loop where Windows fails, attempts to restart, and fails again repeatedly.

Disable Driver Signature Enforcement

Allows drivers containing improper signatures to be installed.

Start Windows normally