About once a year, Microsoft releases an update to Windows XP.
These updates contain all the fixes and enhancements which have been made available in the previous year. The updates (called Service Packs) provide convenient, all-in-one access to the most up-to-date drivers, tools, security updates, patches, and customer-requested product changes.
The latest Service Pack for Windows XP—Service Pack 2 (SP2)—is all about security, and it’s one of the most important service packs ever released. It provides better protection against viruses, hackers, and worms, and includes Windows Firewall, Pop-up Blocker for Internet Explorer, and the new Windows Security Center.
Windows Update: http://windowsupdate.microsoft.com/
Windows XP Service Pack 2: http://www.microsoft.com/windowsxp/sp2/default.mspx