"Access Denied" to the Admin on Old and New Install
Are you login as an admin. I mean UID = 1 ?
Try to reset the super user password and loggin again.

DRUPAL 6 and below: UPDATE users SET pass = MD5('givememypasswordback') WHERE uid=1;
DRUPAL 7 - https://drupal.org/node/1023428

Also check error logs if you think its an internal permission issue.