<p>I'm looking for a word processing component for .NET that would act like an embedded MS Word in my WinForm/WPF .NET app.  The main goal being for users to be able to create rich formatted text.  I don't really want to have to write a highly custom RichTextBox component.  Any suggestions?</p>

Obviously I have no idea what your particular project requirements are, but
ScintillaNET
is a fabulous control and is flexible enough to do a lot more than just syntax highlighting.  I can foresee cases where it certainly wouldn't be appropriate, but it is at least worth investigating.  After a fair amount of searching it has perfectly filled a void in an application of mine where I need to do extensive contextual highlighting of prose.
Edit
I'm revoking this comment because I've thought about it a minute longer and the finite number of styles that Scintilla provides would make it inappropriate for rich text editing.  However, it is still an excellent solution for any situation where you can determine programmatically how styling will be applied.