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.
This link may also help you?
Windows Vista Solution Center
Windows Vista no longer starts after you install an earlier version of the Windows operating system in a dual-boot configuration