Well...
... Windows XP and Windows 7 use different types of boot loader architechture so you won't be able to use the technique I showed in this article to undo your dual-boot.
Chances are that Windows XP was installed first and then Windows 7 installed second, Windows XP would essentially be the boot loader host and since you can't boot into Windows XP, I can't gaurantee that you'll be able to safely remove it. In fact I would say that this is one of those cases where the old adage "if it ain't broke, don't fix it" would come into play.
Unless of course you are willing to scrap you entire setup by reformatting your hard disk and reinstalling Windows 7.
