As far as I know, if you want to have a dual boot system on a single hard drive, then you must install them in order from the oldest to newest.
I believe your only option at this point with Win7 already installed is to add a second boot drive made bootable to XP.
I am not absolutely certain of this so you would be smart to go to Microsoft's website to do a bit of research or ask their tech support, or check this article;
http://support.microsoft.com/kb/919529
Best of luck and cheers!