Make sure you are beside the phone 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, I want to telephone a customer service rep to activate Windows ". Once selected click "Next" to continue.
Windows will now create an installation ID...
.Next a menu will appear, select your country from the drop down menu...
A selection of numbers for Microsoft will appear, usually one will be a Toll free number the other you have to pay for. Dial the number of your choice.
An automated voice will answer the phone, at step 3 an installation ID will appear, nine blocks of six digits, the automated voice on the phone will ask you for these a block at a time, input them using your dial pad on the phone.
The voice will now give you a confirmation ID once it has the complete installation ID, write this down on a piece of paper, you can use it again if you ever reinstall Windows. Then input it into blocks A to G, once done click "NEXT"
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.