So i have to upgrade from my very old OS, i want to buy windows 8.1 and be able to upgrade parts of my PC when i want, or format C: and totaly erase what is on HDD periodicaly and reinstal OS.

As far as i understood there is some OEM version of windows which does not allow all that. Or maybe it was only for windows 7 and under and OEM has been abandonned?

Anyway i would like to know if i buy this can i do all what i want with it? http://www.microsoftstore.com/store/msusa/en_...s81_011415

Also i have 3 computers at home, if i ever want to instal on 3 PC do i need to buy this x3 ?

Thanks to anyone who might help me with this.