How to resolve the file not found error

I've seen this before, and without knowing the full setup of what you have installed as far as .NET apps on the server goes, *be careful*... and read this first...
http://geekswithblogs.net/paulmehner/archive/2005/08/03/48970.aspx

But I think you probably just need to reinstall or re-register the .net 2.0 framework in iis... If you have more than sharepoint on here, then it may mess up those mappings in the virtual directory, but I remember this worked for me when I installed .net 3.0 before .net 2.0... (I highly recommend you make a backup of sysstate or take a vm snapshot before trying this)

command below will uninstall it...
C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\aspnet_regiis.exe -u


then just install it again...
C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\aspnet_regiis.exe -i

The full list of parameters for "aspnet_regiis.exe" is here...
http://msdn.microsoft.com/en-us/library/k6h9cz8h%28VS.80%29.aspx

Re-register the .NET framework 2 .0 as mentioned above aspnet_regiis.exe -i

That will help you fix the issue.

Are you guys sure of that? As .net Framework 3.0 uses Asp.net 2.0 in it's web applications. What I'm seeing is that one of your base classes (Serialization) is pointing to version 3.0 of that dll. Check the web.config of the administration website.

I did uninstall and re-install .net framework 2.0 but that did not resolve the issue. I now get page not displayed for sharepoint site and central admin site.