If you can mandate Office 2007 (internal project), there's a pretty good API (http://msdn2.microsoft.com/en-us/library/bb739835.aspx#ManipulatingWord2007OpenXMLFiles_Overview). 

If you need to support earlier versions, you could purchase a control such as Apose.Words ($899).

If that's not an option either you're stuck with COM, and instanciating the app on the server.  This again would have to be an internal project, as it requires Microsoft Word to be installed on the server, and PIAs installed (http://www.microsoft.com/downloads/details.aspx?FamilyId=C41BD61E-3060-4F71-A6B4-01FEBA508E52&displaylang=en)

From there you can use the ApplicationClass.Documents.Open, and DocumentClass.SaveAs to save the file as html, and a filestream to read it to a string.