http://i52.photobucket.com/albums/g8/Nureese/Oblivion2011-10-2222-49-27-85.jpg

This pic illustrates it. Black area shows the size of my screen and what the graphics occupy. The game menus behave as if they are on a screen that occupies that area with the scroll background. This creates the misaligned menus, because the graphics show otherwise. It affects the main game menus, esc menus, and the character menus with the map and everything. Game is unplayable this way. What has happened today that caused this? I have not installed any menu mods.

Today, this is my activity with Oblivion ( most recent is on top and goes in order downward ):

1. Added new map loop.bik and Oblivion iv logo.bik. Replaced these files with originals and problem persists.
2. Installed RPG-Blackdragon's Low Poly Trees. Moved those files to a temporary folder and problem persists.
3. Double-clicked Streamline_v3.0_-_Open_Beta_4-10400, which is a self-extracting file which extracted to a temporary directory. Could this have modified my Oblivion install?
4. Replaced Oblivion - Meshes.bsa with my .bsa file that has had Architecture, Clutter, Dungeons, Furniture, Oblivion, and Rocks folders PyFFIed. Replaced the original .bsa and problem persists.
5. Added music files to appropriate music folder. Some of them have long names to identify the download. I don't see what the problem is with this.

Problem is still there with no mods loaded.

Additionally, Oblivion has reverted to a default setting with medium textures, medium tree fade, etc.

I don't know why it has become like this. Is there a fix, or shall I make preparations for a long reinstall procedure?

I think that's your Oblivion.ini, one of the two with 'border' is for menus and the other is for the invisi-walls. Setting the right one back to 1 should fix it, IIRC.

You're right that in was in the .ini file. The problem was under Display, an item called fDefaultFOV=90.0000

I had changed it to 90, in hopes that using it would replace my action of typing fov 90 in the console every time I played Oblivion.

Changing it back to fDefaultFOV=75.0000 fixed it.

That setting does not do what I thought it would do.

From TweakGuides:
fDefaultFOV=75.0000 - Determines the default Field of View whenever you start Oblivion. If set differently from the default of 75 degrees, can put screen elements out of synch and also insert borders around the screen. Use the FOV Console Command instead.

Now there is another problem. Game options won't save.

After setting texture size to large, water reflections on, blood detail high, etc, and restarting Oblivion, the settings have returned to default each time.

After you have changed your settings in Options, open your console, type saveini and hit Enter. That should force the game to preserve your changes.

Thank you both. Whatever it was that changed my options settings back to default is another mystery.