"cannot create the window" message when you open SQL SSMS or Visual Studio

So, on our production machine, we started getting the error "cannot create the window" when most users start SQL Server Management Sudio (SQL 2012) and/or Visual Studio (VS2010).

Short version, Permissions to Machine.Config in C:\Windows\Microsoft.Net\\Config were changed when we installed the AS400 OLEDB drivers (IBM System i Access for Windows V6R1M0), and almost nobody had rights to the file (the only person with effective permissions was me, apparently because I installed it)

I reinstalled .Net,.. No joy... Reinstalled the drivers... no joy... Then I did the most obvious thing I can think of - Give "Everyone" read access on the file itself (right-click, properties, security).

Lo and behold, it works.