string url = "Default2.aspx?ID=1&cat=test";
string fullURL = "window.open('" + url + "', '_blank',
 	'height=250,width=280,status=yes,toolbar=no,menubar=no,
	location=no,scrollbars=yes,resizable=no,titlebar=no' );";
Button1.Attributes.Add("OnClick", fullURL);