In a terminal window, or by pressing Alt+F2 (and selecting the option run in terminal)
copy and paste the following command:
$gconftool-2 --set /apps/metacity/global_keybindings/panel_main_menu --type string "Super_L"
And voila. Pressing the Windows Key will now open up your Applications Menu.
Links:
[1] http://pulse.veltsos.com/category/it/software/software/os/linux
[2] http://pulse.veltsos.com/category/it/tweaks