If you notice in this very editor that I am writing in right now, when you press Ctrl + K and then paste your code and save it, it will become formatte
For my blog, I use Manoli's C# formatter.
The source code of the formatter is available.
Basically you collect all the keywords, compiler directives...etc. and use regular expressions to find them. Then the author wraps HTML tags around them to format the text.
You could follow the same approach and include your own formatting which is compatible with whichever Windows Forms control you want to use.