I'd rather use some existing application for spell checking / grammar checking. Here on CP, there is an article on using Microsoft Word's spell checker (see
Using Word's spellchecker in C#[
^]), the grammar checker could be integrated similarly.
Also OpenOffice should offer an API for the grammar checker (see e.g.
http://wiki.openoffice.org/wiki/Grammar_Checking_API[
^]), but the integration of OpenOffice from a .Net application is often complicated.