Windows do use CPU mark to recognize the system which was installed. Changing sole the CPU will make impossible to boot the windows unless you replace with exact CPU. Sometimes when install a new CPU , you can recover windows install using original install CD and key , but most of times, it did not recover the system and a new install should be done (even losing the serial installed, so call microsoft before doing that if necessary). My advise, install a new HD with new OS , and use the original HD as second HD or copy all the files in a backup disk or put the HD in a storage enclosure to trasfer the files, recover is something very hard to do and depending on your changes (intel to AMD or vice-versa) it will be impossible...