A Windows XP product key is required to activate the Windows XP operating system. The key verifies that the version of Windows XP running is genuine.
Those who use trial versions of Windows, which do not come with a product keys, are often unable to use all the available features and support, and generally these programs won't boot after 30 days. Software companies use product keys to help reduce piracy, and the Microsoft activation procedure adds one further layer. It pairs the product key to the computer at the time of install. This helps protect the end user's key from being stolen by ensuring it only works with the hardware it is originally installed on.