Why this Win API serial port access doesn't work under W2k?

I need to write to and read from the COM port. I have the following code which works under Win98, but fails under Win2k. What on earth I'm doing wrong?

Thank you for you help,
Levente

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function TComport.Open(const ComPort: String): boolean;
var DCB : TDCB;
begin
   Handle := CreateFile( PChar('\\.\' + ComPort),
                         GENERIC_READ or GENERIC_WRITE,
                         0,
                         nil,
                         OPEN_EXISTING,
                         FILE_FLAG_OVERLAPPED,
                         0);

   DCB.DCBlength := sizeof(TDCB);
   GetCommState(Handle, DCB);
   DCB.BaudRate := CBR_9600;
   SetCommState(Handle,DCB);
end;

procedure TComPort.Close;
begin
  CloseHandle(Handle);
end;

procedure TComPort.WriteStr(const Buffer: string);
var WriteBuffer:  array [0..15] of char;
    CharsToWrite, CharsWritten: cardinal;
begin
   CharsToWrite:= length(buffer);
   move(Buffer[1], WriteBuffer, CharsToWrite);
   WriteFile(Handle, WriteBuffer, CharsToWrite, CharsWritten, NIL);
end;

procedure TComPort.ReadStr(var Buffer: string; const CharToRead: Word);
var CharsRead: Cardinal;
    ReadBuffer: array [0..15] of char;
begin
  ReadFile( Handle, ReadBuffer, CharToRead, CharsRead, NIL);
  Buffer:= ReadBuffer;
end;