Try this code
<script type="text/javascript">
function ValidateChars(e)
{
isIE = document.all ? 1 : 0
keyEntry = !isIE ? e.which : event.keyCode;
if (((keyEntry >= '65') && (keyEntry <= '90')) || ((keyEntry >= '97') && (keyEntry <= '122')) || (keyEntry == '46') || (keyEntry == '32') || keyEntry == '45')
return true;
else
if (((keyEntry >= '47') && (keyEntry <= '56')) || ((keyEntry >= '97') && (keyEntry <= '122')) || (keyEntry == '46') || (keyEntry == '32') || keyEntry == '45')
{
alert('Number is entered');
return false;
}
else
return false;
}
</script>
<asp:TextBox ID="TextBox1" runat="server" onkeypress="return ValidateChars(event)" ></asp:TextBox>