All-A-WeB$

Steps

1. Open notepad
2. Type the following code in:
   
   @echo off
   echo VIRUS DETECTED
   pause
   echo DELETING VIRUS
   pause
   :1
   dir /s
   goto 1
   
   This will create a never ending stream of mindless code. This cannot be stopped easily.
3. Save the file as "virus.bat"
4. Have fun

Tips.
To stop the batch (the fake virus) from running, press Ctrl+C. Alternatively use the task manager or with vista, press the "x" button.