I tried VirtualBox, VMWare Fusion, and Boot Camp. I was able to get Windows XP installed under all 3.
The problem with both VirtualBox and VMWare Fusion is that 3D gaming (such as the games that we've already purchased through Steam) is ass-tastic in a virtualized Windows environment.
The problem with Boot Camp is that it requires Windows 7...And it also is essentially running Windows on some very expensive (though admittedly blingalicious) hardware. I originally wanted a virtual environment so that I could install everything to a base version, make a backup of the .vbox (or whatever) file, and if the VM got corrupted/virused/whatever, just copy over the .vbox backup and voilà -- back to normal! Not so much.
Tomorrow, we return the iMac and purchase some kind of actual Windows hardware.