Use Ms Word vía ActiveX in ASP.NET it's a bad bad idea. It's possible, but currently, this way is not supported by Microsoft, in fact (http://support.microsoft.com/default.aspx/kb/257757).
You will have troubles with performance, scalabity, user's word versions.... And you will have a lot out-controlled winword.exe processes, and so on.
For this reasons, I use a Silverlight Rich Edit Control, from Devxpress. I had a license already, so I didn't try other solutions, like Telerik's Richtextbox, or Google Docs.
Embed editable MS Word document on web page
Edit: I remembered that another way it's using WebDav. I mean, force to client MS Word to open a word doc in a WebDav location. Maybe you can consider it.
How to create a HTML link which forces MS Word to edit document on webdav server
This is the addin for IE:
http://www.microsoft.com/es-es/download/details.aspx?id=15123