Hi,
Possibly this http://support.microsoft.com/default.aspx?kbid=811320

>On a specific server, after a while, it could be days and weeks, the server stops responding to whatever relates to .NET. In our case, it was a Web Service. The server indicate that the server is unavailable. The event viewer says:
>
>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.
>
>With a IISRESET /RESTART, everything works back to normal.
>
>Any advice?