function validateForm()
{
  var theForm = document.forms[0];
  for(var i=0; i<theForm.elements.length; i++)
  {
    if(theForm.elements[i].getAttribute('required')=='yes')
    {
      if(theForm.elements[i].value=='')
      {
        alert(theForm.elements[i].getAttribute('invalidatemessage'));
        theForm.elements[i].focus();
        return false;
      }
    }
		
		if(theForm.elements[i].name=='txtEmail')
		{
			num1 = theForm.elements[i].value.length;
			num2 = theForm.elements[i].value.indexOf("@");
			if (theForm.elements[i].value.indexOf("@") < 3 || 
				  theForm.elements[i].value.lastIndexOf(".") < num2 + 3 || 
				  theForm.elements[i].value.lastIndexOf(".") > num1 - 3)
			{
				alert("Uw email-adres is ongeldig.");
				theForm.elements[i].focus();
				return false;
			}
		}
  }
 
  return true;      
}