How do I redirect the default web site to /exchange ?

1. Log onto the OWA server

2. Open the IIS Manager

3. Open the properties for the default web site

4. Under the Home Directory Tab, do the following

a) Choose redirection to a url
b) Type /exchange in the text box
c) Check the “directory under this one” radio button

5. Click “apply”, “OK”, and exit the IIS manager.

6. Test the changes by opening a web browser, and connecting to the www root folder.  It should now take you directly to the Exchange Logon screen.

Source

http://exchange.mvps.org/owaredirect_frames.htm