Microsoft has decided to relax its Windows Genuine Advantage (WGA) scheme on its Windows XP operating system to match what is currently being offered on Microsoft Vista.
The refreshed version of Windows XP in the form of SP3, due later this year, will give users up to 30 days before they’re required to enter a product activation key. At the moment, users have to enter the activation key in the midst of the installation process for Windows XP. Failure to do so will halt the installation.
The new scheme allows you to enter your product key after the installation. Note that failure to enter your product key after the 30-day period will lock you out of the computer until you furnish the key.
You can read more from this Microsoft white paper.