Speaking of screenshots and desktops, next time you want to prank your friend, take a screenshot of his desktop. Then set that as his desktop background. Then right click and hide desktop images. Now, it looks like everything is normal, but nothing can be interacted with.

For extra oomph, open task manager and close explorer.exe. This will disable the task bar, right clicking, and the start menu. Now the only way to fix it is to know the hotkey to open taskmanager, or to reboot the system.

Also, Google fake windows update screens and then full screen them. Windows is notorious for forcing updates and taking forever to install them. With any luck, your friend will be staring at a perfectly usable computer for hours before realizing. They’re unlikely to want to restart the computer as well, since that(if it were a real update) could corrupt the system/make you restart the install from 0%