Parser Error - Could not load file or assembly

When I changed one of my clients web site from 1.0.61025.0 into 3.5.0.0 - I get the following error on Client's Live server.

Parser Error Description: An error occurred during the parsing of a resource required to service this request. Please review the following specific parse error details and modify your source file appropriately.

Parser Error Message: Could not load file or assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.

Assembly Load Trace: The following information can be helpful to determine why the assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' could not be loaded.

Version Information: Microsoft .NET Framework Version:2.0.50727.4963; ASP.NET Version:2.0.50727.4955  

Solution:

1. Changing the version from 1.0.61025.0 into 3.5.0.0 in the web.config file. 2. And make sure .Net framework 3.5 is installed on the Server/Computer.