I have a Solution...
Add two text box like this..
<asp:textbox id="TxtCountry" onkeypress="change()" runat="server"></asp:textbox>
<br />
<asp:textbox id="Txtcity" runat="server" enabled="False"></asp:textbox>
Then set
enabled=false property of TxtCountry.
After that just write the javascript to that page like below..
<pre lang="Javascript"><script type="text/javascript">
function change() {
document.getElementById("Txtcity").disabled = false;
}
</script>
Don't forget to add script manager to the page...
I hope you satisfied with this solution...
Have a good day..