If you are running a dual-boot setup with more than one Windows operating system, It's pretty easy to configure the\ default boot operating system with the GUI interface. Note that if you are using Windows 7 or Vista as one of the operating systems, you will need to make this change from 7/Vista, not XP.
To do this, right-click on the Computer icon and select 'Properties'. Next, click on 'Advanced System Settings', then click on the 'Settings' button under 'Startup and Recovery'. Now, just select the operating system you want to use.