I tried like this it's working fine.
<script type ="text/javascript">
function Keypress_Event() {
var objTxtBox = window.event.srcElement;
var isOk = false;
isOk = (event.keyCode == 8 || event.keyCode == 46) ? false : true; event.returnValue = isOk;
}
</script>
asp:TextBox ID="txtTest" runat="server" onkeydown="Keypress_Event()"