you want something like this I think
<script language="JavaScript">
NS4 = (document.layers) ? true : false;
function checkEnter(event)
{
var code = 0;
if (NS4)
code = event.which;
else
code = event.keyCode;
if (code==13)
document.myform.submit();
}
</script>
<input type="text" name="User" onKeyPress="checkEnter(event)">
Hope that helps
GL