Taskbar problem

I really don't know what causes this problem and I have seen it before. My
friend asked me few weeks ago if I could help when she noticed that her
taskbar would not work properly. When she opened any programs she could not
see the bar of that application on the bar menu.
The only way to go through open applications was through TAB+ALT. You
couldn't simplify click on it on the taskbar because it had disappeared.
Now, it is my turn. I just uninstalled Windows Messenger and after a system
reboot, I got that problem. It is pretty frustrating. I really don't know if
it' due to a virus or it is just windows that is malfunctioning.
On my taskbar I still can see my clock, the start button and can even put
all what is on the toolbars.
The only thing I cannot see is the current window of the application
running. How can I solve that one?