you can launch a process of IE using the System.Diagnostics.Process.Start("http://www.url.com") but as for filling out the form, I'm not sure this can be done but do not quote me.
You can however embed a WebBrowser control in your application, if you have .NET 2.0, and implement the document_completed event then look through the documenttext/page property for tags and modify them but this would be terrible as you would be parsing strings and what not - nasty stuff.
you can SendKeys to the application, IE in this case for example, and enter your details on the form but you would have to know what keys to press (simple) but you need to be specific for each webpage.
possibly use an anonymous email forwarding service if he is worried about links between the addys
Actually, ahmedilyas second method (that of using a WebBrowser control) might not be that bad.
Instead of trying to parse the DocumentText, you can actually use the Document object. You can then get access to all the HTML elements on the page that the browser has already parsed for you. By getting the Forms collection, you can find the form used for login and then find the username and password textboxes.
While this could be different for every site, there are usually some common things to look for such as element name contains "user" and element type is textbox. This will do a pretty good job of finding the controls that you need to autofill. This is basicaly how a browser tool bar's autofill function works.
After adding a WebBrowser control to the project, you will access the WebBrowser.Document property. The Document.Forms property will expose a list of HTMLElement objects which are all the forms on the page. Then you will look through the Form.Children collection of HTMLElements for the needed textboxes.
That's about as specific as I can get without knowing exactly what pages you're trying to parse. As stated previously, you'll have to experiment a little until you figure out which elements you need and how best to locate them.