Strange MMSYSTEM error with XP

I'm trying to send a GM reset command to a midi device, which works
well in Win7 but not XP (same code). See details below in the code. I
have no clue what could be wrong. Any ideas? It's independent of the
(not) sent data and it happens on my laptop as well as a friend's
computer. Not on my desktop machine with Win7.

THX,
Hans

void CMainFrame::SendGmReset(HMIDIOUT hMidi)
{
  static const BYTE GMreset[] = {0xF0, 0x7E, 0x7F, 0x09, 0x01, 0xF7};
  SendSysex(hMidi, GMreset, sizeof(GMreset));
}

void CMainFrame::SendSysex(HMIDIOUT hMidi, const BYTE *pBuf, int
count)
{
    MIDIHDR mh = { 0 };

    mh.lpData = (LPSTR) pBuf;
    mh.dwBufferLength = count;
    mh.dwBytesRecorded = count;

    MidiErrorMessage (midiOutPrepareHeader(hMidi, &mh, sizeof(mh)));

    // The call to midiOutLongMsg(...) returns 7 and causes the
    // error message:
    // There is not enough memory available for this task.
    // Quit one or more applications to increase available
    // memory and then try again.
    MidiErrorMessage (midiOutLongMsg(hMidi, &mh, sizeof(mh)));
    MidiErrorMessage (midiOutUnprepareHeader(hMidi, &mh, sizeof(mh)));
}

void CMainFrame::MidiErrorMessage (MMRESULT nError)
{
    if (nError != MMSYSERR_NOERROR)
    {
        TCHAR errText[512];
        CString str(_T("MIDI: "));

        midiOutGetErrorText(nError, errText, _countof(errText));
        str.Append(errText);
        AfxMessageBox(str);
    }
}