Open programs icons not visible in task bar

under task bar property see notification zone click personnalise and set the one you want to see

I actually see this on my work PC (windows XP). I've never really bothered with it, but sometimes a browser session or a some running app will disappear when I minimize it. To fix it, I just alt-tab to the program, and it pops back up on the task bar as it brings the window back up.