Came yesterday in email from Microsoft downloads. The link installs a kernel which enables XP Service Pack 3 to automatically download. Mine is downloading and installing right now.
Read the particulars before you click it.
Windows XP Service Pack 3 Release Candidate 2 via Windows Update
Windows XP Service Pack 3 Release Candidate 2 is now available to the public. Specific registry settings will allow you to download SP3 via Windows Update.