running windows in a window on leopard

I don't think there is any free such software, except maybe "Q", which is pretty alpha right now. For reliability, you're pretty much stuck with paying for Parallels, VM Fusion, or Crossover.

 

The terms you're looking for is running Windows with virtualization instead of running it natively, which would be how it runs with Boot Camp.

The two most popular programs are VMWare Fusion and Parallels Desktop. I'm currently running Fusion, and plenty of people run Parallels. From what I've seen, neither is really a "clear" winner over the other. Both have their strengths, so I would suggest going to each products site and comparing the two to decide for yourself which one you like better.

Also, I've seen the Apple store with deals on one or the other usually, so check around before you decide if price is a big issue.

 

VirtualBox is freeware, but I don't think it runs on Leopard yet though...