Romeo - 12:39 pm on Apr 18, 2008 (gmt 0)

You could look at the source code of myBB to learn how they authenticate users, i.e. with what algo they use to do the password encryption, and then just do the same in your own login code likewise.
You may also think about re-using an existing forum user's session instead of an additional log-in, in case a user is already signed in to the forum.