function numTxt_KeyDown(e, eventCtrl) {
            var e = e ? e : window.event;//IE:event
            var keyCode = e.which ? e.which : e.keyCode; //IE KeyCode

            if (keyCode == 38) {
                //UP Arrow
                if (!isNaN(eventCtrl.value)) {
                    eventCtrl.value = Number(eventCtrl.value) + 1;
                    return false;
                }
            }
            else if (keyCode == 40) {
                //DOWN Arrow
                if (!isNaN(eventCtrl.value)) {
                    if (Number(eventCtrl.value) <= 0) {
                        eventCtrl.value = 0;
                    }
                    else {
                        eventCtrl.value = Number(eventCtrl.value) - 1;
                    } 
                }
                return false;
            }
        }

<asp:TextBox ID="TextBox1" runat="server" onkeydown="return numTxt_KeyDown(event,this);" />