//Voila la fonction...

function OpenCenterPopUp(file){
var Left=window.screen.width/2-175;
var Top=window.screen.height/2-175;
//Videment faut modifier la config!!
var Configuration="'toolbar=no, menubar=no, location=no, directories=no, status=no, resizeable=no, width=400, height=350, left=" + Left + ", top=" + Top;
window.open(file,'NomDeMaPopUp',Configuration);
}