Problems with pop-up windows

Hi,
   my name is Piero, I'm 18 and I'm italian. I've got a little problem:
when I open a pop-up window, I can decide where to position it by x and
y coordinates, but, how can i positioned it on the perfect center of the
page?
   Thanks,
   Piero

http://digilander.libero.it/pierofix