If you're willing to make the investment, VMWare Fusion is great for that. Though of course you still have to obtain a copy of Windows afterwards. I've addressed this by taking my ancient, ancient Windows laptop and running a program provided by VMWare that creates an image from your physical machine. (I've forgotten its name at present).
To make the creation process go faster, I usually restore the machine to factory defaults. That way, the registry is smaller and everything is much smoother.
