I have one more suggestion.
You can use a third party utility to edit the Xp boot option out of the bootloader.
Neosmart technologies created an application to easily edit the Vista bootloader called EasyBCD. They have been working on a release for Win 7 and have a beta version out.
I downloaded the beta version and after playing around with it, it appears to be fully functional.
You can download EasyBCD beta for 7 if you wish but you have to go through a short registration process first.
If you want to proceed visit the link below and register:
http://neosmart.net/forums/register.php?do=signup
Use a valid email address as you will need to open an email and click on a link to complete the registration.
Next visit the link below and sign into your account:
http://neosmart.net/forums/showthread.php?t=642
You can now download "EasyBCD 2.0 Beta, Build 82"
Once downloaded install the application and launch it.
Look at the sentence at the top of the opening screen, if it says something like
"There are two entries in the Windows bootloader" click on the "Add/Remove entries" button on the left. Highlight Xp in the existing entries on the right then click on the "Delete" button right above that has become red.
Click the Save button then exit out of the application and reboot.
Hopefully now your dual boot option doesn't exist anymore. You can uninstall easyBCD if you have no need for it anymore.