Windows key, the Super key between Ctrl and Alt is used for launching Main menu in Windows system and Ubuntu using Alt+F1 instead.
If you’d like Super key as keyboard shortcut to launch Gnome Main menu in Ubuntu, follow this simple tutorial.
1.) Press Alt+F2 to launch “Run Application” box, type in gconf-editor and click Run.
2.) Navigate to apps/metacity/global_keybindings/, in right click to set value of “panel_main_menu” to “Super_L”