Easily Set Default OS in a Windows Vista and XP Dual-boot Setup

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 Vista as one of the operating systems, you will need to make this change from Vista, not XP.

First, you'll need to right click on Computer and select Properties:

Next, click Advanced System Settings

Now click on the Settings button under Startup and Recovery

And just select the operating system you want to use:

Easy stuff.

PLAN B : bcedit command from command prompt

When you install Windows Vista as a dual-boot with your regular Windows XP partition, Vista is always set as the default OS. If you want to set XP as the default OS instead, there's a quick command you can run to change it back.

First you'll have to open an administrator command prompt. Type cmd into the search box, and then hit Ctrl+Shift + Enter to open it in administrator mode. You should be prompted for the UAC dialog.

Enter in the following command:

bcdedit /default {ntldr}

Example:

C:\Windows\system32>bcdedit /default {ntldr}
The operation completed successfully.

Now when you reboot, it will set XP as the default.

Source :
http://www.howtogeek.com/howto/windows-vista/easily-set-default-os-in-a-windows-vista-and-xp-dual-boot-setup/