You need to get a registry editor, like PHM Registry - Smartphone Edition.
Launch the program and navigate to:
HKEY_CURRENT_USER-ControlPanel-Sounds-SystemStart- Sound
Change Windows(backslash)Piano.wav to Windows(backslash)*none*
Remember to include the asterisks and backslashes where noted.
HKEY_CURRENT_USERControlPanelSoundsSystemStart Sound




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