Can Browser Helper Object be used to make an "online" ui-automation tool

We are going to make a test-automation tool which resides in the browser(like an开发者_C百科 online version of Quicktest Professional). We build most of the part using html+ajax. the problem is web application can not talk to the user's native machine's resource. Thus it is impossible to automate the user's native application. We come the idea of using IE extensions like Browser Helper Object but knows nothing about it. Can anyone tell me is there restriction on Browser Helper Object accessing native windows api and COM component or is it as freely as WinForm applications?

I would highly recommend you check out Watir:

http://watir.com/

Watir, pronounced water, is an open-source (BSD) family of Ruby libraries for automating web browsers. It allows you to write tests that are easy to read and maintain. It is simple and flexible.