Name = "check"

<script>
//change two names below to your form's names
document.forms.Form1.check.checked=false
</script>

<script language="JavaScript" type="text/javascript">
<!--
function ValidateForm1(theForm)
{
if (theForm.check.checked==false)
{
   alert("Please check the \"Checkbox\" field.");
   return false;
}
return true;
}
//-->
</script>

Page html BETWEEN HEAD TAG

Form html AFTER TAG

Form html INSIDE TAG

onsubmit="return ValidateForm1(this)"