Well, as far as using phpbb, you'd have to consult their faq's for information about using their database information, my guess is probably not, because then they'd have to give you more access to their code, and then you could hijack their program and re-write it. Also, dreamweaver isn't necessarily what you use to make the login form. You need a server side language to work with databases. You could write your own php, asp, jsp, or if you want ease of use, you might want to run Coldfusion, since you're using dreamweaver anyway. You do however have to pay for the Coldfusion software. You will be able to develop in coldfusion but you won't be able to host with the free version.
the development version is here:
you also have the option to buy it. Who is hosting your site, or do you host your own site?