Hi quack. In XP follow these instructions.
To specify the default operating system for startup
Open System in Control Panel.
On the Advanced tab, under Startup and Recovery, click Settings.
Under System startup, in the Default operating system list, click the operating system 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 you want the list displayed before the default operating system starts automatically.
Hope this helps.