Take a screenshot of the victim's current desktop, set it as the desktop wallpaper, and then hide the icons and taskbar. At first, everything will look normal, but nothing will respond to any mouse clicks. Another approach is to take a screenshot of their desktop, set it as their wallpaper, move some icons around, and take another screenshot. Repeat until you have a very cluttered desktop, with only a few icons that actually work and the victim is left to search to find what's usable.

10. Plug a wireless mouse/keyboard and control a computer from afar

Back in the day, there used to be trojan-like (but benign) VNC programs you could run to mess around with people but those no longer work. Alternatively, installing something like TeamViewer is an elaborate option for a prank but one that will take most planning than most.
However, if you are in an office setting with desktop PCs, just plug a wireless mouse and keyboard receiver in the back of the prankee's computer, and control it from your desk. The receiving person will be lost when the mouse starts moving by itself, starting applications, and typing weird messages on the screen. Combine this with one of the pranks above and you're in for a good laugh.