We're gonna need some more specifics to help you. First of all, is this XP or Vista? SP2? SP3? Media Center Edition? Using legit licensed software that's not OEM? The other thing to look at is whether you formatted the Boot Camp partition when installing Windows, because if you're using XP you must select either NTFS or FAT, you can't choose "Leave current file system intact", and Vista won't even let you proceed without selecting NTFS. I'd recommend taking a look over http://manuals.info.apple.com/en_US/Boot_Camp_Install-Setup.pdf to make sure you've performed the steps correctly.