You're unlikely to have anywhere near the same hardware complement on Windows Machine A and Windows Machine B. And, much Windows software is licensed to the machine on which it was initially installed (CPU S/N, disk drive S/N, MAC address, etc. -- all things that don't port to a new machine).
And, that still doesn't address the "I've got new hardware, do I want to keep running with old versions of this software?"
By far, the easiest machines for me to update are my *BSD boxes -- pull drive, install in new machine. Done.