I have Windows XP on... Everything, it seems. 2 Top-range i7s and lots of early Pentium 4s, pretty much just because it worked and was supported for so long.
Granted, I dual-, triple-, and quad-boot all of the more modern things, with my favorite being my main Ivy Bridge i7 with MS-DOS/Windows 3.11 (Really!), XP, 8.1, and OS X just to see if it was possible (it is) and if the Windows 8 bootloader could handle all of that (it can).
I do keep a dedicated XP machine around for older software/hardware compatibility reasons, though. It's mainly for working with some old software, and isn't used that frequently.