Just happened to be reading about this subject today at
http://www.nytimes.com/packages/html.../circuits.html

An excerpt:

....One solution is a program called Parallels Desktop for Mac ($80,
Parallels.com). It lets you run Windows *in a window*, right on top of
Mac OS X, so you don't have to restart when you want to run a Windows
program. You can copy and paste between Mac and Windows programs, and
you save a lot of time. Parallels can run any version of Windows back
to version 3.1 (and even DOS, Solaris and so on), and it's about 90
percent as fast as running a dedicated Windows PC...

-Tom