When you ran NBI the 2nd time, you should have unchecked the option to install Chameleon (you had already installed it the 1st time). When Chameleon is installed, it breaks existing dual-boot capability unless you jump through some hoops. I know how to fix this problem on Windows 7, but XP is a little different. If I were you, this is what I would try. Please keep in mind that I have not tried this and it may not work. You may end up not being able to boot Mac or Windows and have to reinstall. With that said, and at your own risk, try booting your XP install disc/flash drive then entering the XP Recovery Console (press "R" to enter the console once you get to the installer "Welcome" screen). Once in the console, select your Windows installation and first try running the "fixboot" command. You MAY have to first set the XP partition as active using the "diskpart" command. I'm not sure exactly how to do that on XP, but the "help" command should tell you. Once you have successfully used "fixboot," make sure the Mac partition is active again then reboot. If dual-boot still doesn't work, repeat, except this time try using the "fixmbr" command instead of "fixboot."
Please post your results back here.