disable enter key

<asp:TextBox onKeyPress="return disableEnterKey(event)" runat="server" name="txtUserName" ID="txtUserName" MaxLength="20" Width="135px"></asp:TextBox>

function disableEnterKey(e) {
var key;
if (window.event)
key = window.event.keyCode; //IE
else
key = e.which; //firefox
return (key != 13);
}