I'm a Rails dev, and some of the Cappuccino Rails concepts look damn compelling. Can I develop Cappuccino apps on a Windows (Vista) machine?
Believe me, I know how frustrating it is to ask "how do I do this on my OS" and somebody says "use a different OS". As a linux user in a Mac-centric Rails community, I've heard it before. But you're going to face a lot of challenges with Ruby and/or Rails on Windows.
I think there's a good compromise, though. Install Ubuntu Linux (free) as another partition on your hard drive. The Ubuntu install makes it easy to carve out a small piece of your hard drive for itself, whatever size you want. In fact, every time I get a new laptop with a Windows install I really didn't want, I install Ubuntu on the majority of the drive, but leave Windows on a small chunk. I paid for the license, might as well keep it.
Once you have Ubuntu installed, Rails work will get a lot easier, and you don't need to give up Windows to do it. I think the dual-boot option is a good one.
http://www.ubuntu.com has everything you need.