Im hoping this person comes back to clarify things, i was actually looking forward to reading the outcome of this, seems like a pretty cool idea.
Make one of those old school car alarms "Protected by Viper, stand back" when someone approaches my room. seems simple enough though, since my computer is always on, just use 2 Arduinos some Xbees or even cheaper RF links , a motion or laser"type" of detector and send the signal to a listening Arduino at my computer and blast the sound

, because in my house knocking is not a habbit :/