Yes mate, that solution is never going to work because, at least as far as my limited understaning applies, the hardware confiugration which that particular hard drive is expecting to see (which was written during Windows install) is now totally different.
Best solution - buy a new hard drive, put in your sons machines and install a fresh copy of Windows on it.
Without a LOT of additional info about your sons machine it's difficult to formulate a complete plan.
Desktop/laptop
Hard drive configuration
OS
RAM etc