Go with Windows 7 Pro and XP Mode
I'd buy a PC with Windows 7 while you still can. If you get the Professional, Enterprise, or Ultimate, you can get the free XP Mode which runs as a started task under Windows 7. You can't have XP Mode with Windows 8 by the way. If you buy one with the Home Premium version of Windows 7, you can either use the any time upgrade to the Pro version or install virtualization and your own copy of XP.