I recently installed .Net Framework 2 on my 2k3 box which already runs Plesk 8.1.1 .
When I try to run .aspx pages I see the following error:

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.

The page will run properly if I set "Local System" as the identity of the application pool instead of default identity which is "Network Service" but this configuration is very dangerous for server's security.

I reviewed application event and I noticed the following message:

Failed to execute request because the App-Domain could not be created. Error: 0x80004005 Unspecified error

I searched on the internet, and I've done any recommends that I've found in forums, and most of them were related to permission issues.

I even tried FileMon on the server but I didn't notice any file access exception!

The exact version of .Net running on my server is : v2.0.50727
I've installed .Net 2 along with .Net 1 and sites using different versions are under different app pools.

desperately w8ing for recommendation from u guys.

I also got this error, with Plesk 7.6 on Windows 2003. In my case it was because the user used for the application pool identity (in IIS) didn't have sufficient permissions to access the content of the specific web site. I gave sufficient permissions for the IWPD.... user (with Windows Explorer) and the problem disappeared. Looking at your error code it seems to try to do something database related.