In Visual Studio, "Help > Manage Help Settings" you can download the contents to disk. Then you can press F1 to view them.
This feature is even available in Visual Studio Express. You can start the help server manually at %PROGRAMFILES%\Microsoft Help Viewer\v1.0
Installing the Windows SDK and the .NET 4 help contents took ~700MB for me. There's probably no way around it.
(Windows SDKs used to contain the documentation but it seems like they've stripped it out in v7.1)
You can even do a search offline in the Help, though from my experience the relevant rate is worse than Google.
Edit: The "Microsoft Help System" is also available in the SDK so you don't need to install Visual Studio to use it