Specifying the Default Operating System for Startup

http://support.microsoft.com/kb/306559

Specifying the Default Operating System for Startup

If you have more than one operating system on your computer, you can set the operating system that you want to use as the default one for when you start your computer:

• Click Start, click Control Panel, and then double-click System.
• On the Advanced tab, under Startup and Recovery, click Settings.
• Under System startup, in the Default operating system list, click the operating system that you want to start when you turn on or restart your computer.
• Select the Display list of operating systems for check box, and then type the number of seconds for which you want the list displayed before the default operating system starts automatically.
  
  To manually edit the boot options file, click Edit. Microsoft strongly recommends that you do not modify the boot options file (Boot.ini), because doing so may render your computer unusable.