http://blogs.msdn.com/jpsanders/archive/2009/06/11/infopath-form-and-other-xml-office-documents-do-not-open-from-internet-explorer-raw-xml-displays-instead.aspxI came across this problem. The key to this was that the documents opened fine when clicking on them, so I knew the Office installation itself was fine. Some rogue application on install or removal had apparently removed the appropriate Content Type registryen-USCommunityServer 2.1 SP1 (Build: 61025.2)