How to retain control value?

This is my code

<asp:ScriptManager id="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<Triggers>
<asp:AsyncPostBackTrigger ControlID="CheckBox1" EventName="CheckedChanged" />
</Triggers>
<ContentTemplate>
<asp:CheckBox ID="CheckBox1" runat="server" AutoPostBack="True"
oncheckedchanged="CheckBox1_CheckedChanged" />
<asp:Label ID="Label1" runat="server" Text="Label" ViewStateMode="Enabled"></asp:Label>

<input id="Button2" onclick="call()" type="button" value="button" />
</ContentTemplate>
</asp:UpdatePanel>

My javascript
<script language="javascript" type="text/javascript">
function call()
{
document.getElementById("Label1").innerHTML = "balaji";
}
</script>

I'm changing my label value on Button2 click event in java script. Now if i click check box, the label value goes to default one..

i can't retain label value on "AsyncPostBack"..How to retain?