LONG lRes = RegOpenKeyEx(HKEY_CLASSES_ROOT,
    "mykey", 0L, KEY_WRITE, &hkSub); 

if(ERROR_SUCCESS != lRes){
    lRes = RegCreateKeyEx(HKEY_CLASSES_ROOT,
        "mykey", 0, "", 0, KEY_READ | KEY_WRITE, NULL, &hkSub, &dwDisposition);

    if(ERROR_SUCCESS != lRes) return;
}

lRes = RegSetValueEx(hkSub, NULL, 0,
    REG_DWORD, (BYTE *)&key_num, sizeof(int)); 

if(lRes == ERROR_SUCCESS) MessageBox("등록 성공"); 
else MessageBox("등록 실패"); 

RegCloseKey(hkSub);