monitor problems

hi, maybe someone can help me , I'm trying to run a dos program on a pc that does not have a monitor all the times, but when I start my program everything works fine until I unplug my monitor. In other words whenever I unplug my monitor from the video card, the program stops. I've tried winxp,win98, even pure dos. I always run into the same problem.If somebody has any ideas, it would be really appreciate because I'm out of ideas right now.Thanks