HANDLE hMutex;
 hMutex = CreateMutex(NULL, FALSE, "QSoftObjName");
 if (GetLastError() == ERROR_ALREADY_EXISTS)
 {
  MessageBox(NULL, "You can open only one instance of <app name>!", "<app name>", MB_OK);
  CloseHandle(hMutex);
  // The original code in the line above was "ReleaseMutex"
  // which is wrong because this thread didn't create this mutex.
  // Thanks again Torando...15/01/03
  return(0);
 }

 ReleaseMutex(hMutex); // Ofcourse, don't forget to clean up, thanks Tornado for reminding me :)
//BTW- There are a few other ways to achieve this goal...But this is the easiest one IMHO.