The solution is so simple...but so hidden on the web :

  1. Open task manager (ctrl-shift-escape or Start => Execute => taskmgr)
  2. Go to the "Processes" tab
  3. Select the process of Redsn0w
  4. Right click on it, and define its affinity to only 1 Core/CPU. Let say that you have a 4 cores machine, by default CPU0 CPU1 CPU2 and CPU3 are checked, so you just uncheck CPU1 CPU2 and CPU3, leaving CPU0 checked and this will unlock Redsn0w !!