the VSTO forum supports the VSTO technology. As VSTO runs in-process with an Office application, instantiation and release of that application and its objects is not a supported topic, although you may find the odd discussion about it. However, the correct place to ask such questions is the office.developer.automation newsgroup. You'll find a link to that, and other venues for non-VSTO, Office-related questions in the forum's Please Read First message. For this question, I recommend the office.developer.automation newsgroup.
Right off-hand, in your code snippet, I'd say the problem is that you're trying to save a new document. Doing that will trigger the Save As dialog box, blocking Word until that dialog box is answered. Use D.SaveAs and specifiy a file location + name and your code should run...
I very much recommend that you make your Word.Application Visible while developing/testing. Especially if you're not familiar with the object model. That way you can see things like this.
Cindy Meister, VSTO/Word MVP