For this "prank" you will need some sort of a "live operating system" (something you can run off of a flash drive or CD). I recommend either Knoppix (something any computer tech should have in their tool box anyway), or puppy linux, but you can use whatever you wish. I will be using Knoppix in this instructable.
I am not responsible to anything that may happen to your computer because of this "prank", however; I have played around with it on virtual machines quite a bit and I have never run into a problem. If for some odd reason you do run into a problem, please post it in the comments and I will help you fix it.
Step 1: Download and Install Knoppix
If you want to use it on a CD, use ImgBurn to burn the .iso file to a CD.
If you want to use it on a flash drive, make sure the drive you want to use is blank, and use the Windows 7 USB/DVD download tool to put the .iso file on your flash drive.
The operating system will not work if you just burn the .iso file directly to a CD or copy to a Flash drive, it needs a program to unpack it.
Step 2: Boot from Knoppix
Step 3: Create a folder on the "C:\" drive
First you will click the "Knoppix" folder on the desktop. Then you will select the hard drive that your Windows system is on. Usually this is sda1, but make sure you can see the Windows folder so that you know it is the right hard drive. If your victim dual boots, then you might have to do this to more than one hard drive. Then right click a blank spot on the window and select "New" then select "Folder". Name the folder whatever you wish, but remember the folder name for when you are ready to recover the computer. I suggest using "April Fools".
Step 4: Move "explorer.exe"
See the pictures if you need help with this.
Step 5: Time To Disappear!
Click the start"ish" button on the lower left hand corner of the screen, then select "logout", and depending on if you want to shutdown the computer or just restart, select the proper option. Once Knoppix is shutdown, it will prompt you to remove your disc. Make sure to do this so the computer doesn't boot into Knoppix when your victim boots up.
Step 6: The result
Step 7: Repairing the computer
To use taskmgr, press "Control"+"Alt"+"Delete" together, and if prompted click "Start Task Manager". Then click "File", then "New Task (Run...)". In that window, type the path of the process (if you used the "April Fools" folder, it will be C:\April Fools\explorer.exe).
Your computer should look like it normally does now (except for the taskmgr window open... you can close that now).
Even though it looks like normal, it isn't yet. We still need to move the file back to where we got it.
Click "start" then click "Computer" or "My Computer" (depending on your OS... only one will be displayed.) Then double click your "C:" drive. Double click the "April Fools" folder, then right-click the "explorer.exe" file. In Windows 7 you can just move the file, but I'm not sure about previous versions, so try clicking "Cut" first, but if it gives you any trouble when you try to paste, go back and use "copy".
Navigate to the "Windows" folder, then right click a blank spot on the window, and then click "paste". On Vista and 7, the UAC will ask you at least once to confirm that you want to paste. You do want to confirm this.
Now go back and delete your "April Fools" folder, and remove it from the Recycle Bin.
I would suggest that you now reboot the computer.
Step 8: Conclusion
Only use this if you really have to.... (ya right!) ;)