If you're running Mac OS X and would like to run Windows and Windows applications in a virtual environment, you have a number of options, but the fastest in my opinion is VMWare Fusion. And right now, it's the cheapest too: Amazon.com has it for $39.99 (after a $20 mail-in rebate), which is a smoking deal. (Compare Parallels Desktop at $79.99.) Anyway, this is what I use on the Mac and it works great. Combine Fusion with an OEM version of Windows XP Home ($90) or Pro ($140) or various OEM versions of Vista ($90 to $169), and you've got a fairly inexpensive solution for running Windows and Mac apps side-by-side.
Note: This is going to sound impossible, but I think I picked up Fusion for just $19.99 at Fry's Electronics at some point last year. I assume that was a temporary "Crazy Eddie"-type sale.