An anti-piracy scheme created by Microsoft which requires users to "activate" their copy of Windows or Office. Activation sends your
product key and a unique hardware
identifier to Microsoft, so they know that a given copy of Windows is installed on a given PC. If a user tries to install that same copy of Windows on another computer, they are blocked. Activation is usually done automatically over the internet, but can also be done by calling Microsoft and talking to a rep.
Product activation was designed to stop piracy. But the only thing it's succeeded in stopping is the installation of legitimate copies of Windows by users who rightfully own it. Activation often fails when someone upgrades the hardware in their computer (since the hardware identifier is different). People who pirate software can easily get around
product activation by use of cracks and key generators.