Windows 7 does not use the boot.ini file, Vista and Windows 7 use a different system. Deleting Windows 7 won't affect the boot.ini so it will still boot to XP
Now, I can't be 100% certain but I'm close enough to say that "I would do it". If XP becomes unbootable for some odd reason, you can always restore the MBR using the "fixmbr" command from the Recovery Console to boot XP .
Here's the KB page on this, it's actually very simple, but I doubt you'll need to do this. Set up the Recovery Console anyway and print the page if needed. Description of the Windows XP Recovery Console for advanced users