Text Box allowing only alphabets

<asp:TextBox runat="server" ID="txtFirstName" onkeypress="javascript:return CheckText(event.keyCode);"></asp:TextBox>

function CheckText(evt) {
            var keyCode = (evt.which) ? evt.which : event.keyCode
            if (keyCode == 8 || (keyCode >= 65 && keyCode <= 122)) {
                return true;
            }
            return false;
        }