Dear Krileon,
Thank you, the affair is solved. The menu item must be marked like a "Open in a new window with navigation bar".
In order to solve the problem of the forum wide, I did separate menus (for public and register users), left colum for publics, right column for registers. After from module manager, for each public module, in module assignment I did unmark all the menu items for register users. In this way when someone register clicks on "Forum", the forum appears in a new window with the forum and 1 right column.
Regards.