A while ago we integrated Microsoft Word as an embedded control inside one of our .NET applications. Most tutorials recommend the usage of AxSHDocVw.AxWebBrowser (available from the Visual Studio .NET WinForms designer toolbox), which allows simply to navigate to a file-URL pointing to a Word document. The problem is, AxWebBrowser is nothing else then the well known Internet Explorer webbrowser control, so its behavior depends on the client's internet settings. E.g. when "Confirm open after download" has been set, a file download dialog will appear when programmatically opening an Office document (even in case of a local file), and after confirming the document shows up in a new top-level Word or Excel window, not inside the webbrowser control's host (= your application). That's why we switched from AxWebBrowser to the DSOFramer ActiveX control provided by Microsoft, which doesn't have these restrictions.
Most of the time it's not only required to open Office documents but also to define/manipulate their content, e.g. by replacing form letter placeholders with user data. This can be done by applying Word's COM API. It's a good idea not to let Visual Studio generate COM Interop Assemblies on-the-fly (this generally happens when adding references to COM libraries), but to use Primary Interop Assemblies for Office. Those PIAs can then be redistributed together with the application itself. Note that there are different PIAs for different Office versions, e.g. Office XP and Office 2003. Using Office 2003 features on a client which only got Office XP installed will lead to QueryInterface failures. There are two ways to solve this: (A) develop against the lowest common denominator (e.g. Office XP) or (B) build two Office integration modules with similar functionality, but one of them compiled against Office XP PIAs, and the other one against Office 2003 PIAs. It can then be decided at runtime which of those assemblies to apply, depending on which Office version is available on the client. Hence it's possible to take advantage of Office 2003 features on clients with Office 2003 installed, without breaking Office XP clients. Office 2007 is not out yet, but I suppose the same will be the case there as well.