Licensing

[Everyonesfriend01]

The faq says computers are identified motherboard no. and hard disk physical os no. I haven't purchased the license key yet but when I reinstalled windows the trial period started over ( i had completed 25 days out of 100 days). I didnt change any physical parts of my PC so why did this happen? 
Also, If I switch my PC (which i'll be doing after some months) can I use the same license key (deactivate it on old pc) somehow?