I'm using the following:

WriteValue('HKEY_CLASSES_ROOT\CLSID\00020D75-000-0000-C000-000000000046}\Shell\Open\Command', '', 'C:\PROGRA~1\MICROS~2\Office10\OUTLOOK.EXE /nopreview', REG_SZ)

The /nopreview is not added in the registry

Thanks