http://windowsupdate.microsoft.com/
http://update.microsoft.com/microsoftupdate/

If neither of those work, Microsoft has unlimited no-charge e-mail support for updates:
https://support.microsoft.com/oas/de...&wfxredirect=1

If you don't live in the U.S., start here:
http://support.microsoft.com/common/...h=1&gprid=6527

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.NET's service pack 2 and XP's service pack 3 are two different animals.