Windows key when running windows through virtual box on a mac

By default it's the right ⌘ (Command) Key.

The left ⌘ (Command) Key does not work because it is set as the Host (Meta) Key in VirtualBox.

And they can be swapped simply by replacing the Host Key with the right ⌘ (Command) Key, in Preferences > Input > Virtual Machine > Host Key Combination.

On Virtualbox 4.3.6, I use Ctrl + Esc for the Windows button.

Tested on Windows 7 and 8.1

By default the Windows key is mapped to the Command key on an Apple keyboard.

On the Windows 7 Desktop, tap the Command key and see if the Start menu appears.

You can change the mapping of keys in VirtualBox if you don't like the default mapping as well.