The upgrade media will allow you to install without putting XP on first. What you may have an issue with is activation. As far as I'm aware the 7 installer checks the disk for a previous installation before doing anything else so so long as there is an existing activated installation on the disk of XP or 7 with the same serial as you use for the new install when you boot from the upgrade DVD it just works even if you delete the partition or reformat the disk while installing. It will also do the same if you insert the disk when booted from the old install and then launch the Windows installer from it. The install will work even on a clean disk, but you may end up talking to someone in India (and quoting the serial of the XP you are upgrading) to get it actvated.
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