You can build your site around the forum, using the forums tblAuthor table.
All you need to do is include the "includes/common.asp" file within your site pages to connect to the database and do user checks.
Im not sure about the login form, but im sure their is an include for this which you may be able to use - though you may be able to just create your own and point it to the forums login script.
I have a tutorial on how to create
Member-Only Pages, it isnt what your looking for, but if you a competent with ASP it should give you a headstart.
I intend to do a page which will list all of the details you can get for users and their related variables (eg lngLoggedInUserID is the current users Author_ID)
If how ever you already have an existing site, the API which Stardust referred to is what you want.