<p>Has anybody else had trouble with getting the .Net Framework source code?  Google doesn't have anything to say about this error message, and neither does the CodePlex issue tracker.</p>

<p>Here is the command I'm using to get the source code for the modules that make up mscorlib.dll.  Am I doing something obviously wrong?</p>

<p>NetMassDownloader.exe -o source -f "C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\mscorlib.dll"</p>

Do you mean you want to change this system-wide setting, or that you want to be able to override the behavior only in your program?
If it's the latter and you're using the Win32 API, it looks like you might be able to catch the WM_CHANGEUISTATE notification:
http://blogs.msdn.com/oldnewthing/archive/2005/05/03/414317.aspx
I've not tried it myself but it seems feasible.
If it's the former you're aiming for, I've not yet been able to discover a method.