var fieldToFocus;
function computeForm(form) {
  error = false;


if (error == false && isBlank(form.UserName,"User ID"))
  { 
      error = true;                                       
  }


  if (error)
     return false;
    else
     return true;
}


<!-- To check if the input is blank-->
	function isBlank(field,msg)
	{
		if (field.value == "") {
    			alert ("You must enter your "+ msg + " to retrieve the password");
    			field.focus();
    			field.select();
    			return true ;
 		} 
		else 
		{
   	 		return false ;
 		}
	}