Hi, I installed Vista 64bit SP1 and it's running flawlessly. However, I need XP Pro SP3 on the same machine and Bootcamp will not let me make a new 30GB partition on the main drive. My only option is to use another disk but I can't at the moment.
Use a partition resizer from within Windows to add a new partition and install from there. You don't need to use Bootcamp at all, though you might to use the boot.ini method of selecting which Windows to boot.