Use this:
var w = window.open('', '', 'width=400,height=400,resizeable,scrollbars');
w.document.write('Content goes here');
w.document.close();
Instead of:
window.open('Content');
Updated:
Change your code to:
Response.Write("<script>");
Response.Write("var w = window.open('', '', 'width=400,height=400,resizeable,scrollbars');");
Response.Write("w.document.write('Content goes here');");
Response.Write("w.document.close();");
Response.Write("</script>");