Bootcamp: Windows can't install
With the Windows disk in, restart your Mac and as soon as you hear the startup chime hold down the option key and keep holding it down until you get a list of disks/partitions to boot from.  

Pick the Windows disk to boot off of it, then you should be able to point the Windows setup at the partition you created for Windows (be careful you don't over-write your Mac partition!) and install Windows.