IMHO you are getting that error message because the first parameter of

STDAPI CoCreateInstance(
    REFCLSID rclsid,
    LPUNKNOWN pUnkOuter,
    DWORD dwClsContext,
    REFIID riid,
    LPVOID * ppv
);
[download]

is a struct not a string

typedef struct _GUID {          // size is 16
    DWORD Data1;
    WORD   Data2;
    WORD   Data3;
    BYTE  Data4[8];
} GUID;
[download]

defined on my machine in WINNT.H

--blm--