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There is no "deactivation" process for Windows XP. If you want to move your Windows XP installation to a different PC, use these steps:

• Remove Windows XP from its current hardware. You can remove Windows XP by using the "uninstall" routine; if you previously upgraded from another OS and you elected to save uninstall information at the time. You can also format the hard drive, and install an other Operating System.
• Install Windows XP on the other system.
• If 120-days have passed since you previously activated Windows XP, you should be able to use the automatic "on-line" activation method.
• If the activation fails, you will be prompted to call the Windows Activation center. A telephone number will be displayed on your screen. All you have to tell the activation center is that you removed Windows XP from one PC, and installed it on another PC. You will be asked for the 50-digit activation key. No other information is needed. If you are asked if you want to provide your details for registration, you can choose to decline. Registration is optional, and not required for activation.
• The operator at the activation center will give you a 42-digit key, which you type in to activate your new XP installation.

Note: This procedure will be allowed on retail copies of Windows XP. If you have an OEM (Original Equipment Manufacturer) version of Windows XP, this procedure will be disallowed in most cases. According to Microsoft, an OEM version of Windows XP is linked to the specific hardware it was sold on. Some OEM vendors have exceptions to this rule; you have to check with your OEM.

source- helpwithwindows.com