In cases that involve Critical Updates Microsoft's policy is to provide free support and the best thing for you to do would be to take advantage of it.
If I were to guess it is probably because in addition to missing the SP2 update you probably also missed updates for the update process itself and Windows Installer, but in any case I am sure that MS Support most likely has experience in handling this anomaly and should have you fixed up in short order.
Microsoft Security Product Support Policy
Technical support is available from Microsoft Product Support
Services at 1-866-PC SAFETY (1-866-727-2338). There is no charge for support calls associated with security updates.
International customers can get support from their local Microsoft
subsidiaries. Phone numbers for international support can be found