We'll start off with an oldie but goody - making the icons on your coworker's desktop unclickable.

Requirements:

• Time (low) - probably no more than 5 minutes if you know about printing a screen and using an image editor to save it out, maybe 10 if you don't.
• Cost (none) - none.
• Risk (low) - with a pretty low amount of time needed, and no harm to the coworker or computer, it really isn't very risky.

Steps:

1. Get to their computer and make sure all the applications are closed or minimized so that the desktop is showing. If on a windows computer, this can quickly be done by holding the windows key and pressing the "D" key.
2. Click the "Print Screen" button that is usually located above the "Insert" button on the keyboard. This will take a screenshot of the person's screen, thus capturing the desktop and the icons.
3. Open up any image editor you have and paste the screen capture into the application. Save out the image in a desktop background compatible format (usually jpg or bmp will work just fine).
4. Set up the new image to be the backdrop for their desktop. To do this right click on the desktop, click properties, select desktop, then select the newly created image.
5. Move all the desktop icons into some temp directory (so they can be restored later). You should now see the desktop looking absolutely the same, which is the point.

Result:

The result should be a desktop that looks like the icons are still there because the backdrop shows a screen capture with the old icons. Your coworker will most likely try to click on these icons and not understand what the problem is. It should really mess with their head for a good while. If you want to mess with them even further, you can just tell them it has happend to you and that you had to (insert something funny you want them to do here). For instance, say you had to lick the bottom of your mouse to get it to work again. It's doubtful they will fall for that, but if they do please take a picture and post a link to it.