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 Vista returned an error message saying it was incompatible with 64-bit.
Found DOSbox, installed it, and it runs Clipper fine. And yes, there are probably millions of Clipper apps out there still running. Seriously. Every now and then, someone asks me to drag another such application, kicking and screaming, into the 21st century. (Generally I just export the data, manipulate it, then import into whatever the new platform is.)
Trust me, no one writing Clipper apps in 1988 ever thought their apps would still be running in 2009. No one. But a surprising number of large corporations (not to mention countless small businesses) still run mission-critical applications in DOS using a Clipper app someone wrote long ago.