Ah, yes, please stay away from illegal use of such a program. It's actually quite easy to get caught, and nobody wants that.
Another route to take could be video surveillance. My ex-girlfriend recently bought me a Creative Instant webcam (maybe $30) for Christmas, and if I want to, I can set it up so it will start recording once it detects motion. If you're out of town, it can send you an e-mail and you can get a live feed of what's being recorded. Of course, only if the PC in question is connected to the internet at the time. It won't record the person's activity, but you will at least know who, if anyone, is using your computer and at what time.