There are three main ways to Activate Windows XP. Using a modem, phone, or the Internet.

To start the Product Activation, follow the steps below.

Step 1:The Windows Product Activation Wizard is located in System Tools. To open a system tools item, click Start, point to All Programs, point to Accessories, point to System Tools, and then click the appropriate icon., then click Activate Windows.
Step 2: Follow the activation instructions that appear on your screen.

Alternatively, you can open the Windows Product Activation wizard by clicking Start, then Run, and type in "oobe/msoobe /a".