Why do I get the error message ‘Session state can only be used when enableSessionState is set to true, either in a configuration file or in the Page directive’

This problem may occur after you install Windows Sharepoint Server(WSS) on a server that has Microsoft Visual Studio .NET 2003 installed. The WSS ISAPI filter handles all incoming URLs. When you browse one of the ASP.NET Web application virtual directories, the ISAPI filter does not locate the URL path of the folder.
To resolve this refer Session state cannot be used in ASP.NET with Windows SharePoint Services