I just wanted to point out that i think its a terrible idea for software to do bad things to a user's computer or files if it suspects that it has been pirated.
That pretty much goes without saying. There's a pretty big line between scaring the piss out of someone and actually screwing up their computer.
But, like... c'mon... Blood dripping down a white static screen with a translucent, demonic face muttering things like "behind you" over top of radio static noise and a popup asking whether you want to sell your soul? That's some lovely Hallowe'en stuff right there~!
But that's all really "in your face". Why not be a bit more subtle... go after their sanity...
Imagine it's just a programming problem.
1. Discover whether they have speakers or headphones, and what kind?
2. Use 3D audio to play sounds in different locations - to their left. Slightly behind them. Over their shoulders...
3. Schedule playback of different audio in different locations over months with the intensity building slowly.
For the audio, you can use lower resolutions for smaller file sizes and automatically get that old radio static built in to a degree.
Oh... the fun just thinking of it~!
4. Slowly work in video, like TV white noise flashing, or faint overlays of faces or horrific scenes that fade in/out.
5. Schedule the video similar to the audio with increasing intensity.
6. After a year or so have a final climax if they haven't already called in an exorcist~!
And think of what you could do with their webcam! Record bits of their own audio then process it to get useful tidbits to play back. Just how creepy would it be to hear yourself? Disjoint audio that picks out single words to play back? "He... sleeps..." The later, "He... rises"?
In the end, with enough imagination, it would probably be far kinder to simply go after their systems rather than their minds. MWAHAHAHAHAAHA~!