<script>
function submitOnInvio()
{
if(event.keyCode==13)
{
document.getElementById('<%=loginLB.ClientID%>').c lick();
}
}
</script>

<asp:TextBox id="userIDTB" runat="server" Width="152px"
onkeydown="submitOnInvio() "></asp:TextBox>

<asp:TextBox id="passwordTB" runat="server" TextMode="Password" Width="152px" onkeydown="submitOnInvio() "></asp:TextBox>