function openvenster(url,popup_width,popup_height,popup_left,popup_top,scrollbars) { 
                var undefined; 
                if (popup_width == undefined) popup_width=800; 
                if (popup_height == undefined) popup_height=600; 
                if (scrollbars == undefined) scrollbars='no'; 
				
				left_pos = (popup_left == undefined || popup_left == -1) ? (screen.availWidth/2) - (popup_width / 2) : popup_left;
				top_pos = (popup_top == undefined || popup_top == -1) ?  (screen.availHeight/2) - (popup_height / 2) : popup_top;
				
                //left_pos = (screen.availWidth/2) - (popup_width / 2); 
                //top_pos = (screen.availHeight/2) - (popup_height / 2); 
				
                mywin = window.open(url,'popup','height=' + popup_height + ',left=' + left_pos + ',top=' +top_pos + ',width=' + popup_width + ',scrollbars=' + scrollbars); 
                mywin.focus(); 
}