|Download and install MS Virtual PC 2007 or Open Source VirtualBox. Both are free but you need a legal copy of DOS as the guest system. You can install FreeDOS or DR-DOS that are free DOS clones.|
I suggest MS Virtual PC as it is easy to install and manage offering direct access to COM and parallel ports. If you are running Vista Home Premium ignore the warning message while installing Virtual PC: it affects maintenace policies only.
To run just a legacy application you can use a bootable DOS diskette you can find on the web but be aware you probably need a slowdown tool inside the guest environment too. After you complete the setup I can send you such a tool working under plain DOS only NOT a Windows console box.