Boot Camp Assistant for Mac OS X version 10.4 was only available as a beta download. It expired shortly after Leopard, 10.5, came out. So, at this point, I believe your only option is to upgrade to Leopard (or Snow Leopard).
If you already had created a Boot Camp partition and installed Windows, it would still be functional and you could upgrade it to Win 7. It wouldn't require the Boot Camp Assistant.