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.