Sometimes vbox keeps the keyboard even if the user switched focus to other applications.
These step reproduce the problem on my system (Win2000 SP4, vbox 1.3.6):
- have some windows open (notepad , firefox etc)
- have virtualbox started and start one virtual machine (I had windows running inside it, without vbox additions, since it was a new install)
- host Windows task bar is set to "AutoHide" (I don't know if this is important)
- type something in notepad
- move the mouse to the bottom, so that the task bar appears
- click the virtual guest window on the task bar
- wait a few seconds (do not move the mouse pointer away from the task bar)
- click the same button again
now the focus is back in the notepad window, but the keys do not work, the are grabbed by virtual box