Unable to get the filehandle using CreatFile() function cal for write operation

Hello,

I'm using the "CreateFile" function call in order to write to an existing
file.  It should give me a "filehandle".
However, the filehandle I get, is incorrect.  Its 0xffffffff (-1).

I checked the permissions and they're OK(full control).  I'm logged in as
the administrator.
I can read files, but I cannot write to an existing file.

fileHandle = CreateFile(fileName,
GENERIC_WRITE, // open for writing
0, // no share
NULL, // no security
OPEN_EXISTING, // existing file only
FILE_ATTRIBUTE_NORMAL, // normal file
NULL);

Thanks.