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

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