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Microsoft Forefront Client Security regularly downloads updates to the definition files that are used to identify viruses, to identify spyware, and to identify other potentially unwanted software. Forefront Client Security may also periodically download detection engine updates. Microsoft delivers these updates by using Microsoft Update and by using Windows Server Update Service. To manually download the updates, visit the following Microsoft Web site:
Microsoft Malware Protection Center Portal
Read the complete post at http://wmug.co.uk/blogs/cliffs_blog/archive/2010/01/12/ms-kbs-kb977393-description-of-forefront-client-security-definition-updates.aspx