Search for KB936929 to find the standalone "Windows XP Service Pack 3 Network Installation Package".
SP3 was designed to improve security and management without drastically altering the user experience. Notable additions include: windows xp service pack 3 english download
While Microsoft has retired many direct support pages, you can still find the official English (x86) version of the service pack through these channels: Search for KB936929 to find the standalone "Windows