I'm logged on as Administartor.
But maybe lpSecurityAttributes must be set to specified value?
Help tells:
"Windows NT: The lpSecurityDescriptor member of the structure specifies a security descriptor for the new key. If lpSecurityAttributes is NULL, the key gets a default security descriptor."


LONG RegCreateKeyEx(

HKEY hKey, // handle of an open key
LPCTSTR lpSubKey, // address of subkey name
DWORD Reserved, // reserved
LPTSTR lpClass, // address of class string
DWORD dwOptions, // special options flag
REGSAM samDesired, // desired security access
LPSECURITY_ATTRIBUTES lpSecurityAttributes, // address of key security structure
PHKEY phkResult, // address of buffer for opened handle
LPDWORD lpdwDisposition // address of disposition value buffer
);