Hi Fraser,There is a sample ASP.Net page included with the 10.1 version of the viewer that can be used to accomplish this. To utilize the sample:1) In IIS, disable Anonymous Authentication and enable Windows Authentication for the Flex Viewer2) In the deployed Flex Viewer folder, rename default.windows_auth.aspx to default.aspx. As well, delete or rename index.html to ensure it is not used.If you are using ArcGIS Worklfow Manager 10.0 on ArcGIS Server .Net 10, and want to also secure the services using Windows authentication, this KB article can provide more information: http://support.esri.com/en/knowledgebase/techarticles/detail/39408