If you have MS Word installed, you can include the link in the COM library "Microsoft Word Object Library 12.0" in your project. (I think there might also be a .NET interop library, but I have not tested it).
Then in your code you can simply call:
Microsoft.Office.Interop.Word.Application myWord = new Microsoft.Office.Interop.Word.Application(); ... return myWord.CheckGrammar("Your string here");
And that should do it.
source share