Javascript error in Explorer only. Object not defined.?

I get...

Error: 'document.regform.subtot1' is null or not an object

I also get it in another spot... here's what I'm working with..

<SCRIPT LANGUAGE="JavaScript" type="text/javascript">

interval = setInterval("calc()",1);

function startCalc()

{

interval = setInterval("calc()",1);

}

function calc()

{

one = document.regform.subtot1.value;

two = document.regform.subtot2.value;

three = document.regform.subtot3.value;

document.regform.grandtotal.value = (one * 1) + (two * 1) + (three * 1);

}

function stopCalc()

{

clearInterval(interval);

}

dinner_interval = setInterval("dinner_calc()",1);

function dinner_startCalc()

{

interval_dinner = setInterval("dinner_calc()",1);

}

</script>

It works fine in Firefox, but IE gives an error, and the code does not work. Any ideas? Thanks. I know this is a common IE thing, but I can't find a solution that works for me.