Thankx To All I have Done this with the following Code and it works for me...
function isAlfa(evt) {
evt = (evt) ? evt : window.event;
var charCode = (evt.which) ? evt.which : evt.keyCode;
if (charCode > 31 && (charCode < 65 || charCode > 90) && (charCode < 97 || charCode > 122)) {
alert("Allow Only Alphabets");
return false;
}
return true;
}
<asp:TextBox ID="txt_Cat_Name" onkeypress="return isAlfa(event)" runat="server" CssClass="Text"></asp:TextBox>