For all you geeky people out there, this will be old. But if you’re new to this, it’s great to play on co-workers or family members. My mother was not too pleased.
I don’t recommend doing this if your victim is bigger and stronger than you.
First, you take a screenshot of their desktop, and save it. (For more realism, you can even put a program into the screenshot.)
Then you hide all the icons (you can either go into properties to hide them, or put them in a folder in My Documents. Deleting them is not such a good idea. (I learned this by accident.)
Next, you put up the screenshot as their desktop. If you want even more fun, hide their taskbar. Then leave their computer and try to look innocent.
Lots of people will try rebooting several times before realizing.