try this:-
1). When you receive this error, make sure to check the event viewer on the server. This is found under administrative tools. Under the application event log, you will likely find an error that states: "It is not possible to run two different versions of ASP.NET in the same IIS process. Please use the IIS Administration tool to run the application in a separate process."
An application pool is a process that responds to web requests under IIS. An application pool does not have a setting for what type of ASP.Net applications will be run in it. Instead, it loads the appropriate libraries when an ASP.Net application is loaded in the process. Because the libraries for ASP.Net 1.1 and ASP.Net 2.0 are similar, but not the same, the application pool cannot respond to requests for both types of applications at the same time. This can cause sporadic behaviour if you are using the server at the same time as another developer and you have applications using different versions of the framework in the same application pool.
Make sure that on your server there is an application pool dedicated to ASP.Net 2.0 applications and one for ASP.Net 1.1 applications. When you add an ASP.Net application to the server, make sure you select the right application pool for it.
This can happen for several reasons. An error message detailing the cause of this specific request failure can be found in the application event log of the web server. Open the Event Viewer and see the error message reported. Please review this log entry to discover what caused this error.
If the error message reported is:
Aspnet_wp.exe could not be launched because the username and/or password supplied in the processModel section of the config file are invalid.
1). Stops the IIS and ASP.NET state services
2). Restart the IIS service
3). The ASP.NET user no longer has access to the Microsoft .NET Framework folder, the application folder, or the Temporary Files folder.
Refer these links:-
http://vivekthangaswamy.blogspot.in/2009/04/server-application-unavailable-in-iis.html[
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