In April 2014, Windows XP became end of life (unsupported) by Microsoft. As such, it seems that Microsoft are no longer offering downloads of service packs or other updates pertaining to Windows XP. To download service packs for Windows XP, you’ll have to go to another (non Microsoft) website.
These are not ideal as they often contain viruses that can’t even be picked up by antivirus scanners (as they exploit vulnerabilities in XP that are unpatched). The best way to get a copy of Windows XP Service Packs, is to get a copy from someone who kept a previous download of them, so ask an IT buddy.