Update AV signature files on Microsoft Security Essentials
Microsoft recommends that you update the anti-virus and anti-malware signature files before you scan your computer for the first time.
Microsoft Security Essentials is designed to provide home users with basic anti-malware software. It is supported by the Microsoft Malware Protection Center, which updates AV signature files every four hours. It also provides a heuristics function, which analyses malware-like behaviour and sends information back to the Microsoft Security Response Center for further analysis.
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