Humm... I'm not an expert at this stuff so I am probably wrong, but when I had my old forum transfered to my new site, when logging in it would take me to my old site. The reason was that the cookies were set up wrong. If you are able to log in then check the "configuration" section of your phpBB admin panel to make sure the cookie domain is in fact your domain name and not something else.
If that doesn't help, sorry, but as I said I'm no expert, I just use these things I don't really know how they actually
work
[edit: if you install the board throught the Cpanel the the cookies should be set up right already (I would asusme) so actually that probably isn't the problem
]