Parser Error Message: Could not load file or assembly;System.Web.Extensions

Parser Error Message: Could not load file or assembly;System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35; or one of its dependencies. The system cannot find the file specified.
I was facing this problem ,while installing prototype of a web application . Searched through the net and figured out that Ajax was not installed. following steps solved my issue.

1. Downloaded the ajax from http://asp.net/ajax/downloads
2. In the project I added a reference to System.Web.Extensions and System.Web.Extensions.Design

I hope this also helps you.