Microsoft Baseline Security Analyzer
Highly recommended.
The MBSA performs an audit of your computer's security, reporting the problems it finds and explaining how to fix them.
Download it at www.microsoft.com/technet/security/toois/mbsahome.mspx.
The theory behind the MBSA (Microsoft Baseline Security Analyzer) is that if software got us into a mess, it can maybe-just maybe-get us out of it.
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