function codename() { if(document.form[0][checkboxname].checked) { document.form[0][textname].disabled=false; } else { document.form[0][textname].disabled=true; } }

Change dropdown property autopostback to TRUE

change the property of dropdownlist autopostback property to TRUE and try

If you use runat=server then the name will be changed better see in viewsource or/use document.getElementById(CustomerInfo").Checked or try this also document.forms[0]["checkbox1"].checked

Add top and left attributes ChildWindow = window.open('frmCalender.aspx?FormName=' + document.forms[0].name + '&CtrlName=' + CtrlName, "PopUpCalendar", "width=200,height=210,toolbars=no,scrollbars=no,status=no,resizable=no,titlebar=no,top=300,left=350");