Here are some most FUNNY,ANNONYING computer pranks
Is there someone who irritates you regularly and you can't beat them up..Dont't worry here is how you can take your #revenge :)
you just need to sneak in there PC's for two minutes and it will do the magic.
Create a Shutdown Shortcut that Looks like a Folder

This one is really simple to do—just create a shortcut to shutdown.exe on the desktop, and then change the icon to a regular folder, so when they double-click the folder to see what it might contain, they get a nice system shutdown notice instead.
You can change the shutdown time to really far in the future so they won’t lose any work, but mostly so they get the full benefit of your nerd skills.
shutdown -s -t 1925000 -c “System error: overloaded porn folder”
- If they are using XP, the effect is nice and dramatic, and you can even have some fun with the shutdown time… on Windows 7 you won’t be able to show off your geek skills quite as well, but they’ll still see something that will confuse them.

Note: on Windows 7 or Vista you’d also need to disable UAC or else they’ll see a prompt.
Stick a Message Into their System Clock
It is one of the best tricks to annoy someone as most of the people dont know how to switch it back .

You can easily stick custom text into the clock that sits in the taskbar—and most people won’t have a clue how to switch it back.
Just go through the:
Control Panel –> Region and Language –> Additional Settings –> Time..And change the AM or PM symbols to whatever you want.
Activate High Contrast Mode

There’s a built-in accessibility option that can be very confusing to somebody who doesn’t know how to turn it off, and it requires only pressing one shortcut key sequence:
Shift + Alt + Printscreen
That’s it. Once you press that key combo, their desktop will get flipped to the high contrast mode—you can press it again to go back to normal.
The great thing is that even if their computer is locked, you can enable high contrast with the icon on the lower left corner. On Mac OS X, you can press Ctrl+Opt+Cmd+8 to invert the screen’s colors.
THIS ONE IS FAVOURITE OF ALL
Move the Desktop Icons, Take a Screenshot, Set as the Wallpaper

Most people must have done this trick because it’s both simple and irritating, especially for those who are like novice to compter arena.It can be done in a lot of ways,but all follow the same basic principle as below:
Take a screenshot of the desktop with the icons where they are, set it as the wallpaper, and then hide the desktop icons.Move the desktop icons around, or create fake icons, take a screenshot, and then set it as the wallpaper. This way some icons work, and some don’t.Take the screenshot of the desktop, and then hide the taskbar below the bottom of the screen before setting the wallpaper (this only works on XP). Even better: flip the image upside down before doing it.
The final product will look something like the image above, assuming you followed the second idea and created a folder on the desktop that they’d be sure to notice right away. They’ll keep clicking, but nothing will happen.
You can combine this with the shutdown shortcut so when they do figure it out and try and open the folder… It’ll give them the shutdown prank too!
Now you have learnt some pranks....use them wisely on your friends and cousins..HAPPY PRANKING
DID YOU CHECK -