Hey Use AJAX Extension controls .Which are available at
http://ajax.asp.net/default.aspx?tabid=47.Download and install and use it.DOcumentation also there you can download it use it.
coming to your problem I solved using AJAX controls as
<script runat="server">
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
TextBox1.Text = DropDownList1.SelectedItem.Value;
}
</script>
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>Untitled Page</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp
criptManager ID="ScriptManager1" runat="server">
</asp
criptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp
ropDownList ID="DropDownList1" runat="server" AutoPostBack="True" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged"
Width="100px">
<asp:ListItem Selected="True">first_item</asp:ListItem>
<asp:ListItem>second_item</asp:ListItem>
</asp
ropDownList>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
</ContentTemplate>
</asp:UpdatePanel>
</div>
</form>
</body>
</html>
I think this works
Offline 
Unsolved 
so did it work? 
