Fail to load any of the projects in a solution

Yesterday I opened the solution and everything was OK. Today I tried opening it and I'm getting the same error for all the projects:

error : Could not load file or assembly 'Microsoft.Internal.Tasks.Dataflow, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' or one of its dependencies. The file or directory is corrupted and unreadable. (Exception from HRESULT: 0x80070570)

I reinstalled visual studio (ultimate 2012). I tried to reinstall .NET framework 4 but windows 8 won't let me. I verified that on another machine the solution works fine. I deleted everythin and took a fresh copy from the source control - a copy which works on a differrent machine - with the same results. What am I missing here?