As far as I know you should remove all commas from the data your are trying to write.

 Code:
WriteValue("HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Custom Forms\Compose", "IPM.Appointment", "49504d2e4170706f696e746d656e742e436f6e6465636f00", "REG_BINARY")
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