Safe Mode is a diagnostic tool built into the Windows Operating System for use in troubleshooting. To enter safe mode, boot up the computer and immediately press F8 repeatedly. This will invoke the Windows Advanced Options Menu, as shown in the pictures below. Choose Last Known Good Configuration first; if this doesn't resolve the issue experienced, choose the appropriate Safe Mode and press enter. If you are experiencing a stop error which causes the computer to enter a reboot loop, load this menu and choose the option Disable Automatic Restart on System Failure.
For more information on safe mode, please view the following articles in Microsoft's Knowledge Base.