I never did that. And I've never been on the receiving end of one. Yet.

This isn't necessarily a prank, but I suppose it could be:
Since I'm in the computer lab at my school for my 7th hour, there's always a possibility that someone is still logged into the computer I use before I get to class. One week, a girl forgot to log off a few times, so I decided to put a "Read me" text file on her desktop that said "Remember to log off next time." Next day - she didn't log off, so I copied the text file and pasted it to the desktop approximately 120 times so there was no way that she could miss the read me file.

My friends and I used to go to our high school's library and mess with the accounts of people who didn't log off. At first it was simple "screenshot the desktop, set it as the background, and hide the icons" stuff, but then we learned about REALLY nasty tricks. We changed all of the fonts on one account to Comic Sans MS, changed every saved file on one computer to an unsettling picture of Gabe Newell, inverted the mouse, stuff like that. One time we made it so that every key-press or mouseclick popped up a small text file that read "The only place you can truly learn is the streets."
We never got caught because we were "the good kids". They blamed it on a few guys who used to bully me back in elementary school, but they honestly couldn't tell a USB from a headphone jack.

I kinda installed the Kung Fu Panda movie on my enemies computer, then pasted shortcuts all over the desktop.

He had no idea who it was~

I've never done it, but I'm very tempted to invert the colors on my mom's Macbook when she's out of the room.

There's probably been around 5 occasions where I ran across the school library last year (mind you, it was about 10 feet, not across the entire library) to invert someone's screen while they're gone/distracted. Their reaction was truly priceless if they didn't know how to fix it, and I'd usually wait for a teacher to come before casually noticing and strolling over, saying how I knew how to fix it, and how a lot of people keep doing it to other people for fun. I don't believe I was caught once doing it~

Oh yeah, there's been loads of times were I'm turned screens upside down. It's so easy that you can even do it when they are at their computer, just not looking at the keyboard.

There are 3 I constantly love to do. The well known hide icons and screenshot desktop prank I do a lot. I also like to turn screens sideways or upside down, ever since I accidentally did it to myself. Sometimes I'll assign different keys to different letters for people I REALLY don't like.
9th Grade year we used Google Docs for most of our English 9 homework. When someone aggravated me, I would delete everything they wrote and replace it with something retarded. I wouldn't restore it until they begged me to do it.

My friend hates Twilight more than Freddy Mercury had legendary in his entire body. He left his school laptop logged in, so I changed his wallpaper to some steamy scene from the movie and some other twilight shenagins.

Not really a prank, per say. However, during my last year at High School, in my 7th hour Creative Writing class. I would install Google Chrome on any netbook I used since I had the installer saved to my school storage folder thingy. I would usually change the theme/skin on it too. Usually to something random that I liked such as some dark colored theme with planets but then I put one with Rainbow Dash on it. My friend in that class, however, would take it a step further and install an entirely new OS on there which was Linux.

I just messed with all my friends computers at school when they turned away to make their desktop cats...

I don' know, me and my friends had a code not to mess with each others computers unless permitted. So yeah, they said I could....

Sometimes your friend's laptop is just sitting there by it's lonesome... and I can't resist. Once I googled an image of a cracked screen, hid all the desktop icons, and set the image as his wallpaper. You should have seen him sweat trying to figure out what happened!

He didn't even notice that there was nothing wrong even when he opened up the start menu and what not xD