Sometimes modifier keys like Alt, Ctrl or the Windows key get "stuck". I didn't quite work out thewhy but sometimes it happens, as if the OS just doesn't see the key-up event.
Anyway, the usual way to get rid of that is simply by pressing all modifier keys once, so just hit Alt, Ctrl and AltGr and the issue should go away.
It's especially annoying with the Windows key, though, as this causes programs to start on every second keypress (E for Explorer for example). On a slow machine that can be frustrating.