Registration with Microsoft is optional.
Product Activation of Windows XP is mandatory.
There are important distinctions between activating your
Windows operating system and registering it.
Product activation and product registration are not the same.
Activation is completely anonymous; it requires absolutely no
personally identifiable information to complete. Activation is
required and ensures that each Windows product is not installed
on more than the limited number (usually one) of computers
allowed in the software's end user license agreement, or EULA.
During activation, you can also register your copy of Windows.
Registration is not required; however registering your copy ensures
that you receive product support, product update information,
and other benefits. The process registration takes only a few minutes
to complete. Personal information (for example, "contact" information
such as an email address) is required if you decide to register.
You have a 30-day grace period in which to activate your Windows
product installation. If the grace period expires and you have not
completed activation, all features will stop working except the product
activation feature.
--
Carey Frisch
Microsoft MVP
Windows XP - Shell/User
Be Smart! Protect your PC!
http://www.microsoft.com/security/protect/