Is there a Coding Standard document available for download from Microsoft ? I want to use their standards, mainly for C#.
If you wants guidelines published by Microsoft, google them or use links in other answers.
But if you want "standards used in Microsoft's own code" - there isn't a single answer. Microsoft is a big company with many divisions and acquired code, so each project or product has it's own style.
Even style of .NET Framework itself is slightly different from what StyleCop suggests.
My point is, paying more attention to those guidelines and white papers than Microsoft itself does might be not be a good idea.