Re:

Sujit,

I suppose the system might be loading old DLLs at the runtime. Try these steps -

Delete all temporary ASP.NET files, by removing the folders under the following directory: 

    C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\

Your mileage may vary (win directory differences, etc)

Ref Link - http://codebetter.com/blogs/brendan.tompkins/archive/2006/11/15/Fix-for-ASP.NET-_2200_Could-not-load-file-or-assembly-App_5F00_Web_2E002E002E002200_-Error.aspx