Hi,
if you really can login as root - from any terminal - you can run
Code:
adduser $(USER) adm
adduser $(USER) admin
to add the $(USER) to the groups adm and admin. I am pretty sure that one of these groups is the Ubuntu admin group. I think the admin group is the correct one.
If you can not login as root, you have to work with a boot CD - but this would be topic of another posting.
Greet-o-mente