If you are looking for a fun easy prank here is a simple way to remove everything off the screen including the task bar. This wont hurt the persons computer or anything but it might scare the crap out of less than savvy computer user.
Simply open the task manager. You can do this by pressing Control Alt Del and selecting task manager. Select the Process tab. Then click the process Explorer.exe. Click end Process. It will tell you some warning but dont worry as long as they didnt need to save anything you are fine. Everything should go bye bye. Desktop, taskbar and icons. They will more than likley think there computer has crashed or something along those lines.
If you want to fix it you can simply restart or go to the task manager again. This time select file — New Process and type in explorer.exe. Tada all back