visual studio project gives error after rebuilding

I am able to open a saved copy of my project in visual studio and it runs fine. After I rebuild the project it gives me the following error :

Could开发者_JAVA百科 not load file or assembly 'System.Web.Ajax, Version=3.0.31106.0, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e' or one of its dependencies. The system cannot find the file specified. 

this is driving me crazy. please help

Use Fuslogvw.exe to troubleshoot this problem. It shows you where the CLR looked for the assembly.