guy i work with has a problem with his laptop - his taskbar doesnt display any of the programs that are running, nor any of the IM windows or anything like that. its got the clock, and the quick launch toolbar and start button, just it doesnt show any programs..
It could be that the little slider has pushed them all to the right so they are not visible. If you right click on the task bar and unlock it (if it's locked) you might see a slider. Slide it to the left and see if you can now see them.
With all those selected, depending on screen resolution, they may not have any physical room left to display open programs. Try expanding the height of the start bar to give it more real estate on the screen, or deselect some of the toolbars to see if more running programs become visible.
Had one similar to this but not exact. Changed the desktop theme back to "classic", then back again and it started working. BTW, also had one where an installed program was not compatible with SP2 and bar disappeared.