<div>
    <span>Search</span>
    <asp:TextBox ID="txtSearch" runat="server" Width="170"
        onkeydown="if ((event.which && event.which == 13) || (event.keyCode && event.keyCode == 13)) {$('#<%=lkbSearch.ClientID %>').click();return false;} else return true; ">
    </asp:TextBox>
</div>
<asp:LinkButton ID="lkbSearch" runat="server" CssClass="searchButton">
    <asp:Image ID="imgSearch" runat="server" ImageUrl="~/Images/Master/button_go.gif" />
</asp:LinkButton>

Page.RegisterHiddenField( "__EVENTTARGET", lkbSearch.ClientID );

<asp:Panel ID="pnlSearch" runat="server" DefaultButton="imbSearch">
...
</asp:Panel>