If you are running Windows, Ubuntu, other any other Linux distro in VirtualBox under Mac OS X, you are likely to discover that the normal Mac shortcuts keys don't work. command C don't copy, you can't tab using Command tab etc.
It may seem like a buggy VirtualBox build, but it is not so. You just have to remember that host OS behaves like itself, and not like Mac OS X. There is no command key in the host OS' traditional environment.
Here are the shortcut keys for various operations in the non-Mac host OS.
control C Copy
control V Paste
control X Cut
control tab Tab within application
option tab Tab across applications
Tip: Forget the ⌘ key when you are using the host OS. Try control or ⌥ when in doubt.