Taskbar

I've windows XP Pro on my pc,
After installing the sevice pake1 I discovered that when
I open any window doesnt show up on the taskbar even I
opened 10 windows and if I mimimize any of then, I dont
see any on the taskbar.
first, I thought that my windows xp got crashed, so I
reinstalled windows ( repair ) again, and the problem
being fixed, but just I installed the service pake again,
the problem came back.
Anybody faced this problem?
thanks for all.