I'm running XP Home, when i press the above buttons i get the little green and black icon in the tray stating the cpu usage. But the box that usually appears on the screen has changed.
Now it is just a box with no tabs on the top, with end task, switch task and new task at the bottom. So the only part I can is this default tab that appears when you press the mentioned buttons. This is the tab that shows what applications are running in the back ground.
As far as i can tell all other windows are working fine.
I've done full virus scan, checked for errors, and anything else i can think of.
Not stupid at all. As far as I know this is undocumented. If you click on the 'Performance' tab and then double click in the side bar and then shrink it by grabbing and dragging with the mouse then it gives a useful window on activity.