I am getting this error when I run the DNN install version of 4.0.2 with VS express.

everything compiles properly...but it does't run??

I had it working a month ago on same machine??

IIS logs aren't indicating a problem....

 Server Application Unavailable

The web application you are attempting to access on this web server is currently unavailable.  Please hit the "Refresh" button in your web browser to retry your request.

Administrator Note: An error message detailing the cause of this specific request failure can be found in the application event log of the web server. Please review this log entry to discover what caused this error to occur.

I got if fixed but the form does not allow me to post the solution... I do not know why.

I had the same Problem in 3XX and 4XX...

I have XP and SP 2 with all the latest updates + 1.1 and 2 Frameworks
I found this script on the net and even though it says it is for framework 1 only I did it anyway:

 I had to rem one line that checked if you have anything else than 1.1.4322 Installed as I wanted it to run anyway
The line is: if exist %SystemRoot%\microsoft.net\framework\v1.1.4322\mscorlib.dll goto v11_installed
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Look at http://www.asp.net/faq/ms03-32-issue.aspx

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After this script I ran "aspnet_regiis -i" On the Framework 2 Command promt (this is to RE install the Framework) and restarted IIS with "iisreset"

I then Re checked the Security settings (Permitions for ASPNET User) on the directories and All was OK

Hope this help!!!