Open a window on top of another

[Fabio Rotondo <fsoft galactica it>]

Hi,

 I'd like to know what is the fastest method to open a window centered 
on top  of another.
I have some "quick" dialogs that popup on my app window, but at the 
moment they appear
randomly on the screen, while I'd like them to be centered on the parent 
window. Any hint?

Thanks.

Ciao,

  Fabio