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Set xp As Default OS

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 \par The operation completed successfully.
Now when you reboot, it will set XP as the default.