So, the kind of DOS programs that won't run under XP (or NT, or 2000 - and possibly not really 98) are where programmers have been too lazy to use the proper DOS/BIOS APIs, and have just hacked directly into hard-coded absolute addresses.
The issue that remains in XP, even for well behaved DOS ising "the proper DOS/BIOS APIs", is that Mr. Gates does not believe that you need to be able to output things in very precise timings. One standard that has trouble ocasionally is J1708 that states that any gap between characters larger than 2 bit time is end of message.

Erik