Since the underlying XP is full version and can migrate freely, it qualifies you to use Windows 7 Pro upgrade version (which can also migrate freely) on the new rig.
Nor does the XP need to be installed to serve as qualifier - you only need to not use it or use it as a qualifier for any other OS while it is serving as qualifier for the new rig.
To install upon a new HD leave the Product Key blank when first asked, then after install do the quick registry workaround given here to activate Upgrade version on a new or cleaned HD. Clean Install with a Upgrade Windows 7 Version
If the HD has another OS on it, the installer will see it to allow Upgrade version key upfront even if you go on to use Custom install>Drive Options to Delete, Create new and Format partitions.
I've compiled a series of steps to get a perfect reinstall based on hundreds done here - use the ones which apply to your situation: Reinstalling Windows 7
After double in-place Upgrades you should really enjoy the difference with Windows 7 in it's native clean-installed state!