I have the exact same problem. I had a dual-booting Win XP & Vista system, used VistaBootPro2 Beta to change the default to my Win XP install and updated the title for it. After that I got the NTLDR error with Win XP.
I then followed these instructions and (once) after using fixboot to repair XP I used the Vista DVD to fix the Vista boot. It restored everything exactly as it was AFTER I made the changes to the boot menu, including breaking XP.
So then I repeated the process but used VistaBootPro2 to restore the Vista config (after fixing XP) and it did the same thing. So XP still won't boot.
I think the problem is that it is restoring all the changes back to the boot menu, including whatever broke XP in the first place. So now I'm stuck. I'll have to reinstall Vista near as I can tell, unless someone has a better idea. Thanks!


