Except for that whole ".NET framework" thing.
All web sites are of course written in ASP.NET. Giant portions of the Xbox Live service are written in .NET. Heck lots of tools and utilities are also .NET based.
Is someone going to rewrite IE in C#? Not likely, that isn't what C# is for.
C# isn't being used as a systems programming language, but it isn't meant to be one. (That said, I've written high performance code in C# before, you have to know what you are doing and understand your GC, same as writing high performance code in Java!)
Tons of LOB software internal to MS (indeed I'd say the vast, vast, majority of internal LOB software) uses C#. Lots of plugins and extensions to various tools use C#, and I wouldn't be surprised if lots of the PowerShell Cmdlets and Modules are C# based.
Now I have worked on a number of commercial projects that were written in C#, but of course I am unable to discuss them!
(None of this is spoken of as an official MS employee of course, it is not but my own opinions!)
MS doesn't have a unified build environment, every team generally does their own thing. It has its pluses and minuses.
Shared code would occasionally be useful at Microsoft, but not as often as you'd think. Generally when relying on another team's code, it is preferred to take it as a binary drop when they do a product release, just like any other customer. This helps prevents needing to deal with churn in one's dependencies.