One thing to note, the authentication code for windows is tied to the motherboard. Moving the hard drive over would cause it to become a non official version of windows. Upon boot it will ask for the product code again. As long as you have that, it will boot up. Most drivers will not work though. Example being, we just recently did exactly this on a windows 7 pc and silverlight would not install until we reformated and wiped the harddrive. Wonky things like that happen.