Make sure you are connected to the Internet before beginning this process.
To activate Windows XP Professional Click Start, All Programs, Accessories, System Tools, and then click Activate Windows. The "Let's activate Windows screen will appear. Click on "Yes, lets activate Windows over the internet Now". Once selected click "Next" to continue.
You now have the option to register with Microsoft, click "Yes" or "No" and then Click "Next". Registration is not required to activate Windows.
If you clicked "Yes, I want to register" then this screen will appear, fill in your details and click "Next", click "Skip" if you have decided not to register.
Windows will now check for internet connectivity....
After a few moments, if all is successful then the "Thank You!" screen will appear, click "OK" to finish, Windows is now registered and you shouldn't have to do it again unless you have a major hardware change, i.e. motherboard.