I'm not exactly keen on most keylogger programs really, and I've heard stories of people getting fired for installing keyloggers. You may want to get this cleared before doing it. Past that, I'd suggest something like this instead of a program. Assuming whoever this is knows what they're doing, they can usually turn off a program just by killing the process. Should also let you know if this is a remote connection or local.
But I'd also think the question would be who is doing this. If you find that it's local, you can setup a cheap webcam with motion detection to find out who.
Or if the idea is simply to get them to stop, just a little bluff usually does the trick. Setup a small batch script or something to run in the startup folder that opens a message simply saying "Hi! You're being watched 🙂"