In addition to what Ed is telling you (which is all true), the predominate reason why many DOS programs cannot run under XP is that in the Bad Old Days, some DOS programs would reach out into IO memory or video memory directly (comm programs and game programs were the worst two offenders) and tweek memory locations directly.

Starting in Win98 and finished in Win2K, this was no longer allowed. The system ran DOS programs in their own memory space, and DOS programs that try to reach out and touch IO or video memory directly will be slapped down with an access violation.

"Well behaved" DOS programs run just fine under Win2K or XP. Really, they do. "Well behaved" is defined by Microsoft as "using the supplied API's and not trying to reach out and touch devices, memory, the video memory, keyboard, etc directly. Go through the libraries and API's."