Your project sounds too close to the OS for Java developpement. Howewer, you may consider using Windows' system tray to activate your background application with a click. To achieve that, I suggest you to check theses packages :
Yeah, unfortunately Lionel is right on this one. It's not possible to trap the keystrokes unless a container of some sort has focus. I know that you said you can't do native C/C++, but that would be a solution to your problem... i.e. writing some native code to trap the keystrokes and calling this native code through JNI.
Dad always thought laughter was the best medicine, which I guess is why several of us died of tuberculosis. -Jack Handy