SYMPTOMS

When attempting to access web mail or web administration, a message indicating "Service Unavailable" is presented in the browser.

Application log contains:

aspnet_wp.exe could not be started. The error code for the failure is 80004005. This error can be caused when the worker process account has insufficient rights to read the .NET Framework files. Please ensure that the .NET Framework is correctly installed and that the ACLs on the installation directory allow access to the configured account.

Security log contains:

The logon to account: ASPNET
 by: MICROSOFT_AUTHENTICATION_PACKAGE_V1_0
 from workstation: WORKSTATION
 failed. The error code was: 3221225578

CAUSE

This may occur if the ASPNET account (inbuilt ASP.NET identity account) has been reset manually or by some other agent. The credentials for the account then become invalid, causing ASP.NET applications to fail.

RESOLUTION

The solution to this issue is to is to re-synchronize the credentials for the ASPNET account.

Firstly, inspect machine.config file and the contents of the processModel element.
Ensure that the username is set to "machine" and the password is set to "AutoGenerate", as this means that it is set up with the defaults.

Once this has been done, if the error persists, try running aspnet_regiis.exe from the 1.1 framework directory and restarting IIS.  This should synchronize up the ASPNET password.

If it still does not work, then the default permissions for the ASPNET account may have been modified. In this case, review this knowledge base article for information on the required permissions/rights:  
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q317012#4

If your server is a domain controller or backup domain controller, review this article:
http://support.microsoft.com/default.aspx?scid=kb;en-us;315158

REFERENCES

http://support.microsoft.com/default.aspx?scid=kb;en-us;Q317012#4