How can I run FS2004 in the backround??

Well, I know that it is a pretty stuipid question coming from me since I have been using the FS series since 2000, but I took a one year break and I forgot how to make Flight sim run in the backround ::)! I remember that in FS2002 there was an option that you had to check off in the realism setting or something like that. Is there such a thing in FS2004? And if there is...where do I find it 
Thank you very much in advance!!
Vic
By 'run in backround' I mean to have set up a flight, have it on autopilot, and Alt+Tab out of flight sim without having the "simulation paused".

Thank you very much in advance!!
Vic
By 'run in backround' I mean to have set up a flight, have it on autopilot, and Alt+Tab out of flight sim without having the "simulation paused".