You can install Windows without the Boot Camp Assistant. However, if you still want OS X on your Mac, you should use Boot Camp to partition/resize your disk drive. Then boot your Mac --without-- using the Boot Camp Assistant, but press Alt/Option after the startup sound and boot directly from the Windows CD/DVD. Windows will find the Boot Camp partition. Do NOT let Windows change the partition layout of your disk - you can let Windows reformat the Boot Camp partition, but do NOT LET IT PARTITION! (Unless you want to lose OS X, of course.)
After the Windows installation is complete, you can - or rather: have to - install the Boot Camp software in Windows to get all the required drivers.
After you've installed the Boot Camp drivers that came with your Mac or Leopard, you can download the latest Boot Camp drivers from Apple's website and install them. The drivers on Apple's website are an update ONLY, so you HAVE TO install the original Boot Camp drivers first, otherwise this won't work.
I've experienced problems with Boot Camp's disk resizing utility in the past: On certain machines, it wouldn't allow more than 32 GB for the Windows partition, although there was much more space available. I still don't know how to solve this without scratching the entire machine. If you have this problem, you probably just have to suck up this weird 32 GB limitation for Windows.