"XP supports multiple operating system on same computer so you can (could have) two OS running on your computer."

Is that two OS running Simultaneously on the same PC?
Somehow I doubt it; the whole reason XP stops your old DOS direct-port-access stuff from working is to allow multi-tasking and avoid multiple tasks all trying to hack the same absolute IO address at the same time!
How on earth could two completely different OS on the same PC possibly cope with that?!?!?!

The original poster has already dismissed the idea of rebooting to DOS just to do the programming - because then none of his nice Windows tools work!
Seems to want to have his cake and eat it!

The idea of a separate DOS PC sounds by far the best approach - if he really must insist on keeping his rusty old DOS stuff...