ups, sorry, das letzte Anführungszeichen ist noch zuviel ...
function openKP() {
var setTop = (screen.availHeight>600)?(screen.availHeight/2-300):0;
var setLeft = (screen.availWidth>800)?(screen.availWidth/2-400):0;
sym_kp = window.open("datei.hmtl","fenstername","width=800,height=600,left=" + setLeft + ",top=" + setTop);
}