Installing Windows on an external HDD?

Hey
I have a macbook and would like to run Windows on it, too.
Besides partitioning the default hard disk, is there any way to install an OS on another disk?
What I am trying to do is installing Windows on my external hard drive and run it on my macbook. Is that even possible?
thanks in advance

Unfortunately, no. Windows simply isn't designed to be installed on an external hard disk. This is a limitation of Windows, not Boot Camp. You can't install Windows on an external drive on a standard PC either.

thanks! :]

I can run Windows Vista and XP on a Firewire drive - but it's via a cloned copy of Leopard using Parallels (or VM Fusion).

Is there a reason why you don't want to use a program like Parallels?