javascript validation in asp.net 3.5

For email validation you can use following javascript code:

 if (!isEmail(document.getElementById("txtEmail3").value))
 {
   alert("Please Enter a Valid E-Mail Address in Shipping Address");
   document.getElementById("txtEmail3").focus();
   return false;    
 }
//Note: txtEmail3 is the id of the textbox

function isEmail(s){  
            if (s.search(/^[a-zA-Z][\w\.-]*[a-zA-Z0-9]@[a-zA-Z0-9][\w\.-]*[a-zA-Z0-9]\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z]$/) == -1){
             return false;
            }
            else{
             return true;
            }
          } 

now If you have given as "select date" as the first index and if you are trying to enforce user to select valid date.
you can try this:

if(document.getElementById("cboDate").selectedIndex ==0)
{
  alert("Please select the Security Question");
   return false;
}

//Note: cboDate is the id of the textbox

I didnt understand the checkbox part.