What is the best way to set up Y2K Joke on someone's computer?

Copy Y2K Joke to your victim's computer and rename it to something official-sounding like "Y2K_System_Check.exe" if you want extra believability. Run it when they are nearby or about to sit down. The fake diagnostic sequence is convincing enough that they may genuinely worry about their computer -- especially if they remember the original Y2K scare. The moment it asks them to tilt their computer is when the joke usually lands.