Build an XLL that are compatible with excel 2003

I Build an XLL that work fine in excel 2007, but don't work in excel 2003.
I followed all the steps to be compatible with Excel 2007.
I finally reduced the code to:

#include <windows.h>
#include "xlcall.h"
BOOL __stdcall xlAutoOpen(void)
{
return 1;
}
but when i tried add-in in excel 2003 get the message:" is not a valid xll"

may be because I installed Office 2007 on my development pc ?
I use visual studio 2005