To manually install SP3 using the standalone installation package or a CD
Do one of the following:
If you downloaded SP3 from the website, to install it immediately, click Open or Run and follow the instructions on your screen. To install the program later, click Save and download the installation file to your computer. When you're ready to install the service pack, double-click the file.
If you're installing SP3 from the Service Pack 3 CD, insert the disc into your computer with your current version of Windows XP running, and the Software Update Installation Wizard should automatically appear. If this wizard doesn't appear, click Start, double-click My Computer, right-click your CD or DVD drive, and then click AutoPlay.
On the Windows XP Service Pack 3 page, click Next.
Follow the instructions that appear on your screen.
After the installation is complete, click Finish to restart your computer.
If you disabled your antivirus software, enable it again.