When you install Bootcamp and try to use it on an already partitioned hard disk, it will complain telling you that you have to have a disk with only one partition.
If you just need it to burn the windows cd driver, you don’t need to repartition you hard disk.
CTRL+Click on the Boot Camp Assistant and choose Show Package Contents. Now navigate to Contents -> Resources and you’ll find a DiskImage.dmg. Just burn that disk image and it’s all done.