Recently we tried to build an older version of a system that is hosted on Windows 2003 server.  Every project in the build would fail with Visual Studio unable to sign the DLL.  Of course the DLL was not there in the target folder, but that was not the problem.  The build had worked before we rebuilt the server.

A Google search produced a suggestion to change the file permissions on:

C:\Documents and Settings\All Users\Application Data\Microsoft\Crypto\RSA\MachineKeys

We gave Administrators full access to the folders and were able to compile. 

Just a couple of days earlier, I had to give IIS_WPG access to this folder in order for a web service to be able to read the keys and decrypt a WSE message.  The service had worked fine after I had installed it and given access to the key file using the WSE key utility. 

I noticed a pending Windows update icon was no longer in the system tray.  It appears that update had tightened security on that folder.