Help!!!! I do not know what happened, but folders in Nautilus over the sudden decided to open in a new window. So, before I reach the wanted folder I have 50 windows opened!! It's a total disaster. How, where can I change it? I turned up side down whole Nautilus and found nothing at all!

look in gconf editor under apps/nautilus

Edit->Preferences->Behavior and check "Always open in browser windows"

Edit->Preferences->Behavior and check "Always open in browser windows"
THis is unckecked.
gconf -- is it an xml file?
I found it in home/linuxfond/.gconf/apps/nautilus

the gconf.xml has nothing about browsing .. :-(

Nautilus till recently had nice buttons on the left side: open with... etc.
Now this is replaced with the tree. I do not want that tree, and I want to open all folders in the same window.

If I run slocate gconf I find one thousand of different gconfs

Well, have no idea if that's what I need, but here it goes:

<?xml version="1.0"?>
<gconf>
<entry name="confirm_trash" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/confirm_trash"/>
<entry name="enable_delete" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/enable_delete"/>
<entry name="show_icon_text" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/show_icon_text"/>
<entry name="show_directory_item_counts" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/show_directory_item_counts"/>
<entry name="click_policy" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/click_policy"/>
<entry name="executable_text_activation" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/executable_text_activation"/>
<entry name="theme" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/theme"/>
<entry name="show_image_thumbnails" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/show_image_thumbnails"/>
<entry name="thumbnail_limit" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/thumbnail_limit"/>
<entry name="directory_limit" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/directory_limit"/>
<entry name="preview_sound" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/preview_sound"/>
<entry name="show_special_flags" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/show_special_flags"/>
<entry name="sort_directories_first" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/sort_directories_first"/>
<entry name="show_desktop" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/show_desktop"/>
<entry name="desktop_font" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/desktop_font"/>
<entry name="desktop_is_home_dir" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/desktop_is_home_dir"/>
<entry name="search_bar_type" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/search_bar_type"/>
<entry name="window_always_new" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/window_always_new"/>
<entry name="background_set" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/background_set"/>
<entry name="background_color" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/background_color"/>
<entry name="background_filename" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/background_filename"/>
<entry name="side_pane_background_set" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/side_pane_background_set"/>
<entry name="side_pane_background_color" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/side_pane_background_color"/>
<entry name="side_pane_background_filename" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/side_pane_background_filename"/>
<entry name="default_folder_viewer" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/default_folder_viewer"/>
<entry name="hide_built_in_bookmarks" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/hide_built_in_bookmarks"/>
<entry name="sidbar_width" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/sidebar_width"/>
<entry name="start_with_toolbar" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/start_with_toolbar"/>
<entry name="start_with_location_bar" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/start_with_location_bar"/>
<entry name="start_with_status_bar" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/start_with_status_bar"/>
<entry name="start_with_sidebar" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/start_with_sidebar"/>
<entry name="side_pane_view" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/side_pane_view"/>
<entry name="add_to_session" mtime="1118279667" schema="/schemas/apps/nautilus/preferences/add_to_session"/>
</gconf>

Is it edible?

Yes, so, check it! You are misunderstanding the option. What it means is that it will open a browser window instead of a single folder (spatial or whatever name you like) window.

Sorry, my Nautilus is 2.4.0 and there is no
Preferences->Behavior and check "Always open in browser windows"

Instead, there is "Open activated item in a new window" (under "Behavior" tab)

This is unchecked, so I do not see why Nautilus opens everything in a new window.

Sorry, for some reason I was assuming a more recent version. 2.6 is the first Gnome I've used. Maybe you could just delete your entire Gnome config. When my Gnome is messed up I always delete (after backing up ofcourse) the ~/.gconf* dirs.