Recent release of FastCGI Extension 1.5 for IIS 6.0 and IIS 5.1 has several new features and improvements that make it easier to run PHP on Windows and IIS. However, all the new features are only available when using Windows XP or Windows Server 2003
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Read the complete post at http://blogs.iis.net/ruslany/archive/2010/02/05/fastcgi-module-differences-across-iis-versions.aspx