The Pikachu worm mass emails itself and then adds the lines "del C:\WINDOWS\*.* and "del C:\WINDOWS\SYSTEM\*.*" to autoexec.bat, which is executed at next boot. The virus wasn't well coded and this deletion routine actually prompts the user if they want to delete the directories. I chose no (and made a sarcastic remark, too :p).
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Email-Worm.Win32.Pikachu or Pikachu is a worm on Microsoft Windows that spreads through email...