Would be much better if all instances of Windows has a unique number you could lock the exe too.
The catch is that only a fraction of the computers out there have a unique number -- even though the case of your brand spanking new Dell machine has its own windows licence number on it, the pre-installed copy of windows on its harddrive has the exact same key on it as the other three hundred thousand people that bought the same model over the past year.
The serial on the case is only used for future re-installs, out of the factory the windows install is a complete clone.