You dont' need to reinstall - If you need to repair the boot loader: Boot to your Win 7 DVD and select Install. On the second screen, lower left of center, there is an option to Repair. Select that.
When you installed XP second, what happened is XP over~wrote Win 7's boot manager with it's own. Unfortunatly, XP's boot manager does not know/understand how to boot Win 7. So you must put Win 7's loader back.