You could also use something like VM Ware. But that is just running Windows inside of Linux. But it is FAST. And I do mean FAST. Instead of trying to imulate Windows, it provides a separate environment for Windows to boot, which is cool. You see your machine go through the boot sequence in a window.
You can find out more about VM Ware at www.vmware.com. It is sweet. You can have multiple instances of it running, which would allow different sessions of Windows running at a time. So you could have one session running to develop, and one to test. Or you could develop on NT and test in 9x. It also has network support and other great stuff.

j