You must be able to re-install an OEM copy (I've done it - sometimes more than once) in case of upgrading a HDD or motherboard for instance - or just wanting to wipe the whole thing and start afresh. I've re-installed sufficiently in some cases that I've had to go to the automated telephone activation (when auto online hasn't worked) and even had to speak to a real person once and there still wasn't a problem re-activating.