I have downloaded the unrelated installer for SP3 from the link above and installed it on one of my laptops which is
running Windows XP Home Edition in it. I also considered that it might be also good to offer some of the screenshots while
Installation screen so that you are aware about what to expect. After saving the service pack to my desktop with double-
clicking on it, I was been greeted by the following setup screen suggesting me that I can backup my system first (this is
the same kind of screen that has greeted the users while installing the SP2).

After clicking at the Next, I was then greeted with the license agreement

Next, is the installation window will be prompted out for the finding the saved backup files for (accepting the default location in it)

Then the installation procedure will be carry on to confirm for the essential disk space, backup files, and install SP3

Lastly, the system will ask you do you want to reboot now or later on, I then choosed to reboot immediately. After reboot, the computer will display a blue install screen for finishing the installation process and boot into my profile. It took about several minutes for showing out my profile as the system was finished with the cleanup. Then a couple of DOS boxes emerged out for a minute and the installation was finally complete. The complete installation on a Home edition system also took around about 20 minutes. I have also opened the System Properties window for showing the system which is now been updated to SP3.

Good luck with your update to Service Pack 3!