Windows XP Service Pack 3 (SP3) is a free update for Windows XP. SP3 includes all previously released Windows XP updates, including security updates, hotfixes, and select out-of-band releases. 
Read the Windows XP SP3 overview white paper
Steps to take before you install Windows XP SP3
The easiest way to update your Windows XP–based PC with SP3 is by using the Windows Update. Just set Windows Update to automatically download important updates and Windows Update will download SP3 when it's available for your PC. If you don’t update your PC automatically, download Windows XP SP3 from Windows Update manually. Download Windows XP SP3 now from Windows Update Order the Windows XP SP3 CD
For IT professionals and developers, get an installation package to download and install on multiple computers on a network. If you're updating just one computer, please visit Windows Update or order a CD. Download and deploy Windows XP SP3 to multiple computers Free unlimited installation and compatibility support is available for Windows XP, but only for Windows XP SP3, until April 14, 2009. Chat and e-mail support is available only in the United States and Canada. Windows XP SP3 installation support | 
