Everyone must have wished to do something "hacky" at one point in their lives. Have you ever felt the urge to 'tamper' with your friend's computer while they are away or in the toilet? Forget writing embarassing statuses in his/her Facebook or checking up their history, that's just child's play. These pranks will leave your friends freaking out while you laugh away (secretly, of course)
Faking a Blue Screen of Death
A tech-savvy person will call this phenomenon as the Blue Screen of Death (shortened, BSOD). Your everyday samaritan will call this as the "rage-inducing computer crash". But you know for certain that it is only a screensaver you have just downloaded. Either way, the next thing they would usually do is reset their computer or call for help (this is where you come to rescue). If your friend has an affinity to smash the pc due to rage/stress then don't do it! (I actually know one person who might do exactly that).
Stop Everyone Else From Using That Wifi
This one requires you to bring a laptop with you and a little prep work. Just download Netcut right here and install it before you actually put it to practice. Next time you are in your friend's house, ask him for his wifi password (if he has a password) and go open up the software discreetly, preferably in the corner of the room. When you are done, you should be able to see your friend's bewilderment. For a greater effect, ask an accomplice to watch what you are doing. I personally feel it very satisfying to have that 'control', and your friend will be amazed at what you can do too.
Turn the Internet Browser Upside Down
If you are familiar with a bit of wireless knowledge (IP tables, servers and those things), you might want to try this "Upside Down Trick by Ternet". All you have to do is redirect your target's connection to Ternet's server and should not take more time than a 5 minutes toilet break. You might want to practice this trick first at home before trying it for real. Nothing is more embarassing than a fail attempt, especially ones where you want to look smart.
Turn on the Dvorak Keyboard Layout
The Dvorak keyboard layout is said to be designed to make typing faster and more ergonomic. Fortunately, most of your friends should not know the Dvorak's keyboard layout, making it an easy, harmless computer prank. Let's just say that anything your friends type will turn into complete gibberish. An example being the word, "stupid" into "oyglce". While you're at it, try to actually learn the Dvorak's way since it might be beneficial to you in the future. (the main reason being the risk of Carpal Tunnel Syndrome)
To turn on the Dvorak Keyboard Layout, go to Control Panel--Region and Language–-Keyboards –-Change keyboards, then click the Add button and pick the Dvorak layout.
Hotkey Any Keys Into Any Phrase
Just think about it, which key is the most used key while you are typing? General consensus says it is either the 'space' or 'backspace' key. Using a simple AutoHotkey script with a single line of code is enough for this awesome computer prank.
Note that most of the pranks here is only 'practical' provided that you have physical access to the target's computer and a diversion to the target (toilet breaks, quick stroll, buying something).