<script language=javascript>
  function validate(){
   var email=document.forms[0].email.value;
   var comment=document.forms[0].comment.value;
   if(email==""||(email.indexOf('@')==-1)||(email.indexOf('.')==-1)){
    alert("E-mail field is invalid!");
    document.forms[0].email.focus();
    return false;
   }
   if(comment==""){
    alert("Comment field is empty!");
    document.forms[0].comment.focus();
    return false;
   }
   return true;
  }
 </script>

<form name="contact_form" method="post" action="http://..." onSubmit="return validate()">

<br>
<input type="submit" value="Submit">
<input type="reset" value="Reset">
</form>

 <script language=javascript>
  function validate(){
       var email=document.forms[0].email;
       if(email.checked!=true){
          alert("Email is not checked!");
          return false;
       }
       return true;
  }
 </script>