Often, we have condition when we have multiple forms on a page which have multiple submit buttons as well. Whenever, you press enter on any text box it will call the command button which is at top most sequence of command buttons.
So, to overcome this problem here is the little code. Take your each set of fields and button in a panel and specify the default Button Property. In the example below I have a form which have two buttons one is command button and another is an image button and I am setting the second button of the sequence to get called each time enter key press on the panel. (rather then the top most)
1: <asp:Panel ID="pnl1" runat="server" DefaultButton="ImageButton1">
2: <asp:TextBox ID="TextBox1" runat="server">
3: </asp:TextBox>
4: <asp:Button ID="Button1" runat="server" Text="Submit" />
5: <asp:ImageButton ID="ImageButton1" runat="server" />
6: </asp:Panel>