How do I get Windows XP Service Pack 3
Downloading Sevice Pack 3 Manually
Go to Microsoft Office website. Here the link for your conveniences http://support.microsoft.com/kb/322389#method1
and scroll down a little until you see How to Get Windows XP Service Pack 3. Detail image below.
After you have downloaded it to your computer, double click to install the service pack on your computer.
Downloading Service Pack 3 Automatically
Go to Your Windows Updates and allow the computer to automatically download and update your computer to Service Pack 3