If you've changed the setting from default, log in as an administrator and check the following two items -
Control Panel > Portal Settings > Allow users to turn off the left column?
Columns will always display on the Main Portal Index page.
Control Panel > Portal Settings > Allow users to turn off the right column?
Columns will always display on the Main Portal Index page.
On that same page, there are two other items just above these, which may be of interest to you
