A number of people find that when they want to reminisce about the old days – that they no longer have a working copy of DOS handy – and it is quite hard to buy a copy of DOS at this stage. Well – never fear – FreeDOS may be just what you need: http://www.freedos.org/
FreeDOS is an open source DOS compatible operating system – though you should know that there are still some programs that will only run on MS-DOS. You can download the install .ISO from http://www.freedos.org/freedos/files/
FreeDOS installs just fine inside of Virtual PC – and in my testing was able to run most of my DOS programs. However the FreeDOS installer is very ugly (even for DOS) and a bit confusing (even for DOS) – also – FreeDOS uses an odd directory structure for DOS which takes a bit of getting used to.