This is very difficult to do unless each PC would have the exact same equipment and BIOS (which is VERY rare!). It may start up and run (once!) but then the hardware and BIOS conflicts would begin, up to the point where windows would not even start and run. (I know, I tried!). You can swap hard drives, but it would require a reformat and installation of software to do,
I just built a new PC. Now I want to upgrade my daughter in law's with my old one and my granddaughter's with hers. What are the concerns with simply putting their harddrives into the upgrade computers? These are both older computers with vastly different hardware configurations but we are all running Windows XP.