When an update keeps reappearing it is indicative of some sort of failure, meaning that an error occurred during the installation or a file that should have updated during the installation failed to do so.
Microsoft has a help page to address this very problem and you can find it here:http://support.microsoft.com/kb/910339
Please read and follow the directions step by step according to what best describes your particular situation. You can use the Fix it tool that is provided on that page, if applicable. At the very bottom is a link to contact Microsoft support. Since your problem is related to a security update there is no charge for this service.