These are some good pranks that could be arranged within less than 30 seconds.

Each prank has four parameters:
preparation time(minutes)
arrange time(seconds),
x-time(time in minutes or type of victim activity)
and difficulty(1-3)
1=easy
2=normal
3=hard

preparation time is the time required to prepare the prank before acessing the victim's computer.

Arrange time is the time of acessing the victim's computer needed to get the prank working.

x-time is the time before the prank will go off or something that the victim has to do for the prank to go off.

the instructable is in the april fools contest so please vote!

Step 1: Mouse-off Prank

preparation time= 0.5 minute
arrange time= 10 seconds
x-time= when the victim uses the mouse
difficulty= 1

this is a very simple and effective prank.
the victim has to have an optical mouse for that.

Just take any sticky non-transparent thing(a sticker, a piece of masking tape etc ) and seal the "eye" of the mouse.(see pic #2)
you can also stuff a little bit of paper into the sensor of an optical mouse(pics 3&4)
by doing this you replace the moving (relative to the mouse) table top, or a mouse pad by a piece of sticky thing that is still(again, relative to the mouse), so the mouse dooesn't move the pointer.

Step 2: Some Naughty Little Applications

preparation time=1.5 minute
arrange time= 10-30 seconds
x-time= depends on the type of prank,(see step 4)
difficulty= 1.5-2

These are little programs that can drive the victim crazy if used properly.
I've found mine at RJL software , lizardworks.com and F2ko.de.
The ones I like are
avoid,
replacekeys,
clippy
crazy NCS,
headache
and ButtonShyMouse
just put the folder with the program somewhere on the victim's computer and execute it.
You could even put it on your flash drive and execute it without copying to the computer.

Step 3: Batch!

preparation time=0.5-10 minutes
arrange time= 10-30 seconds
x-time= when executed, on startup, or after a desired time.(see step 4)
difficulty= 1.5-3

Batch is a good thing. You could send someone a message over the LAN, make the victim think there is a virus and even shut down someone's computer!

here are some commands:

@echo off ( @echo on)
makes the commands hide (or show). Put this at the beginning of your batch file.

echo (something)
displays the word "something" in the cmd window, use the "pause" or "ping localhost" command for the victim to be able to read it.

pause
makes the program pause and wait for you to press any key displaying the message "press any key to continue..."

ping localhost -n(time in seconds)>nul
this is used to make a pause for some desired time in seconds

msg * (anything)
displays a dialog window with an "ok" button and a message "anything"

(command)>(filename.ext)
overwrites the file "filename.ext" with output of the command. If the file doesnt exist, it creates the file and writes to it

(command)>>(filename.ext)
the same as the previous one, but it adds the output to the file instead of overwriting it.

(command)>nul
stops the output of the command from showing

del (fullfilepath)\(filename.ext)
deletes the file "filename.ext" in "fullfilepath"

shutdown -(type) -f -t (time_in_seconds) -c "(message)"
shuts the computer down or restarts it depending on what "type" is (s for shutdown, r for restart) in "time_in_seconds" after activation (if you set it to 0, it will shut down immediately), forcing all the applications to close and leaving a message "message" (you could make it shut down without leaving the message by not typing " -c (message) ")

shutdown -a
aborts the shutdown

start
makes a new cmd window

start (fullfilepath)\(filename.ext)
opens the file "filename.ext" in "fullfilepath"

start www.instructables.com
opens the instructables home page in the default browser

using these commands, write a file in notepad(each command on a new line)
and save as something.bat
(don't forget to select "all types" in the lower bar or the file will be saved as "something.bat.txt")
if you want to, you can convert it to *.exe format using the downloadable bat to exe converter or the online bat to exe converter.
if you are using only the commands that do not require victim interaction or viewing the cmd, you can make the app invisible (so the victim doesn't see the command prompt window)

Also, if you want to learn some more, there is plenty of instructables on batch.

Step 4: Changing the X-time

preparation time=0.5-2 minutes
arrange time= 10-30 seconds
difficulty= 1-1.5

so, if you need the prank to start in a different time (not immediately after you set it up) you could use different methods.

First, you have to decide, when it should go off.
I will describe how to perform four options:

on startup,
when the victim clicks on a fake shortcut (good with fake viruses and similar stuff)
after a desired time since activated (using the "time fuse")
or after a desired time since startup.

The startup option is very simple.(difficulty=1)
Simply copy the link pointing towards the prank to the startup folder in the start menu.(see the pic)

the fake shortcut is a little more complicated. (difficulty=1.5) To make it:
1) find the shortcut to the program that is used most frequently
2) right-click on it and select "properties"
3) click the "shortcut" tab
4) change the path in the "target" field to the path leading to the prank
5) click "ok" and you're ready!

the time fuse is relatively simple(difficulty=1):
1) download the archive below the picture,
2) extract the files,
3) run "fuse.exe"
4) set the time before the prank will go off and the full path leading to the prank
5) press enter.

time fuse after startup is a little complicated, as the fake shortcut (difficulty=1.5)
1) download the archive below the picture
2) extract the files,
3) run "x-maker.exe"
4) set the full path leading to the prank
5) press enter.
6) use the "fuse numbers.txt" to pick the right fuse number
7) copy the link pointing towards the fuse to the startup folder in the start menu

Also, I believe in open source, so there's an archive with the files in *.bat format (I converted all the fuses to *.exe to make them run in background. )

Thank you for reading this instructable.