When you’re a Windows user, it is natural if you use the Windows key to open the main menu so that when switching to Ubuntu it seems a bit strange that there is the Windows Key on your keyboard useless there. Here is a command line for windows key binding for the Gnome menu (Ubuntu main menu)
Now open your terminal from the main menu -> accesories -> terminal, and then type the following command in the terminal or copy / paste
gconftool-2 --set /apps/metacity/global_keybindings/panel_main_menu --type string "Super_L"
Now press the windows that is on your keyboard gnome main menu will appear