login.php, misc.php, post.php, register.php, include/functions.php

Hi. I've been playing around with punbb for some good 24 hours. A nice forum indeeed.
The thing I want to do is to make punbb work with my allready working community.
Belive me, I've read up on lot's of post here.
I've read:
http://punbb.org/forums/viewtopic.php?id=8786/
http://punbb.org/docs/dev.html#integration
and almost every topic containing *topic*.

So the problem isn't that I haven't read up. The problem is that I suck.
I sure enough would be able to make a separate login-window on my start-page,
letting the user login with his punbb-login, the be brought back to a specific page, letting
him/her surf around on the page and THEN press "forum" and be logged in.
The thing here is that I allready god a userdatabase for my users, so that
they can access their guestbooks, their presentations, aso.
So I really don't know what to do here. If it is possible for punbb to use the same
login/database as the rest of community, or if it is possible to login to both my community
and the forum - at the same time. One button, 2 functions. Same at the registerpage.
But if I would manage to get that working, what's next? My community uses sessions,
and punbb uses cookies.

Well I'm kinda lost here. If I started from scratch, building my community after my forum then there
wouldnt be any problem. But since I've allready got a working community that's not an option.

IF you know any link that I might have missed, that will describe better to me what has to be done -
please post it. I'm not all afraid of a little reading.

Thanks alot, and thanks for the great bb, and for the great tpl's.
Regards,
Robert.