Unable to read the project file 'YAF.Classes.Utils.csproj'. C:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\Microsoft.CSharp.targets(132,9): The attribute "Returns" in element Target is unrecognized.

Unable to read the project file 'YAF.Classes.UnitTests.csproj'.

C:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\Microsoft.CSharp.targets(132,9): The attribute "Returns" in element Target is unrecognized.

Unable to read the project file 'Intelligencia.UrlRewriter.csproj'.

C:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\Microsoft.CSharp.targets(132,9): The attribute "Returns" in element Target is unrecognized.

.
.
.
an finally

One or more projects in the solution could not be loaded for the following reason(s):

The application for the project is not installed.

These projects will be labeled as unavailable in Solution Explorer. Expand the project node to show the reason the project could not be loaded.