DOSbox is an open source emulator that will run ancient DOS programs in XP and Vista, including Vista 64.
Among my arcane specialties, I sometimes convert and migrate Clipper database programs to modern platforms. Needed to run Clipper on my Vista 64 laptop, but instead, Vista returned an error message saying it was incompatible with 64-bit.
Found DOSbox, installed it, and Clipper runs fine. And yes, there are probably still millions of Clipper apps out there still running. Seriously.