To upgrade the operating system of Windows XP to SP3 in two ways. First: open "start" - "control Panel" - "System" - "Automatic updates". Turn on Automatic updates, your system will be updated. Keep in mind that the entire pack has a size more than 300 MB, so it can take a considerable time.
You can use the other option updates: download from the official Microsoft website the whole package and start the installation. This option is more preferable as a ready to install package updates will no longer depend on the vagaries of the network. Download SP3 for Windows XP here: http://www.microsoft.com/downloads/ru-ru/details.aspx?familyid=5b33b5a8-5e76-401f-be08-1e1555d4f3d4
Note that the system update may lead to the emergence of various problems, so before beginning be sure to save important data. Save not on the partition where the OS is installed. It's better to keep valuable data on an external drive.
Usually on computers with licensed Windows installs updates, after a restart the user will see information about that on his computer it is installed SP3. But if you are using unlicensed version of the OS, automatic updates or install downloaded Microsoft pack may cause the reset of the activation and the message appears stating that you have an unlicensed copy of Windows.
To circumvent this difficulty, you can download one of posted online the "modified" package SP3. However, in this case, there is no guarantee that along with the updates on your computer will not be installed by Trojan horses. Alternatively, you can install official update, then re-activate Windows using the appropriate apps that you can find online. But the right to use the licensed Windows or go to one of the Linux distributions – in the latter case, you will get rid of licensing issues.