1. You cannot login to anywhere in your forum.
If there are any displayed like wrong password ..etc. Just change it, there are few threads here in vb.org let you know how to change it via phpmyadmin, of course you can simple reinstall your vbulletin.
2. You can normally login and see yourself as logged in in the forum or other place. But when you try to go to admincp. It require you login again, and again.
In this case, try to login in somewhere (forum) then go directly to this link
http://yourvbulletinlsite/admincp/index.php it should help.
anw you can try with another browser and see what happens then.