I just ran into this, as I too am installing Vista as a secondary OS.
When you install Vista, you'll be prompted after BIOS to select which OS to boot to. As for making one default, I don't think that's a good idea, but you should be able to go into "my computer/properties/advanced tab/startup and recovery "settings" button... and you can select default OS and time delay.
As for deleting Vista, the easiest way is to delete the partition, and run recovery console from the XP install cd. Once in recovery console, type "fixmbr" and press [enter]. It'll ask you if you are sure... type "y".
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