Automatic Mouse Clicker.
Set a long initial delay time. (The text-box entries for delays or intervals are all in milliseconds. I don't know the maximum value.)
Set it to wait 547 seconds between clicks.
Total number of clicks is your discretion.
The whole program is a single EXE, and maybe you can rename it to something that'll blend in, maybe svchost.exe, or winupdate.exe, something like that.
Use carefully. I don't know how to make it stop once it's started, short of wrestling Task Manager open to terminate the process.
So if you do something like 1 click every 5 milliseconds, repeated 100,000,000 times, you might have some problems.
If they are a touch typer, then switching the keys on the keyboard won't make any difference since changing the caps doesn't actually change the button assignment, and since they aren't looking at the keyboard, they'll just type as normal.
However, if they are a finger typer, then yes that might screw them up.
Except for J and F, with the little locator bump on each one.
Harmless: Set mouse movement to slowest setting; switch left/right buttons.
Best used in combination with another prank, slowing victim's ability to recover.
Psh, if I've got a keyboard, I'll get around Windows.
Then comes a hidden webcam with motion-activated recording.
Fortunately, the worst prank I've had done to me at work computer-wise was tape over the mouse's sensor. I'm not a fan of pranks myself, especially when one can cause the loss of work. Not everyone cares as much about wasting time like that though. Whatever works for you I guess.