If you’re using IIS 7.0 (or 7.5), URL Rewrite is a valuable tool, well worth installing and using. One common use of URL Rewrite is redirecting http://domain.com to http://www.domain.com. Many people are doing this for search engine optimization
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Read the complete post at http://blogs.iis.net/owscott/archive/2009/11/27/iis-url-rewrite-rewriting-non-www-to-www.aspx