Enrique, in template customisation, main menu you could try simply reducing the width of the buttons (and maybe the point size of the text). It sounds like you either have too many webpages to display a heading for each, or each button is too large (so they will not all fit across the screen).

When they won't fit you get the toaster menu.

If you're game to upgrade, you will be pleased to know that menu placement, size and layout is far more flexible in recent editions.