[xmonad] xmonad and VirtualBox/VNC

Hello I've noticed that when a VirtualBox window (in which I am running MS Windows XP) has focus keypresses directed at xmonad (mod key + some other key) are handled by VirtualBox (and the guest operating system it is running) rather than xmonad. The same happens when using a VNC client such as vinagre: keypresses directed at xmonad end up being received by the remote machine I am working with. Is there a way to avoid this and have xmonad intercept these keypresses? thanks ~lara