Error 1 The “Validatexaml” task failed unexpectedly.System.IO.FileLoadException.Could not load file or assembly.

If you are facing the ValidateXaml exception and your build fails then it might be due to the new feature of blocking the downloaded content. This means whenever you download a project from the internet it is quite likely that windows will automatically block its content for few dll’s and if it’s so you will get this error. So the easy solution is to look for the file for which its complaining as in the above solution its the Activity control for which it’s complaining, so just navigate to that file right click on the file and press unblock and that’s it , go  and rebuild your solution and everything will work as desired. I have myself faced this problem in Windows Server 2008 and Windows 7. You might try this also in Windows Vista as well.Hope this helps. Happy programming !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!