I have the Oblivion (version deluxe) from Steam, and I'm having some graphics problem.

In the Oblivion Options (before star the game) when I use the "reset to defaults" the program says "video settings have been set to Very Low Quality, 640x480".

I started play with Ultra High and 1600 x 900, and play ok for two weeks. But now when I try to start the game, it crashes.
I used the "compatibility mode - XP sp3", change the graphics to high but the game only run if I turn off the options "Screen Effects" and all the "distant rendering". If I try to use screen effects ou distant landscape/buildings/trees, the game crashes. And if i run the game and change the value for "distant grass", it crashes, too.

I was played very well for about 50 hours.
Why now I can't?

My pc: Core 2 Duo 2,0 GHz, 4Gb RAM, Radeon HD5770 1Gb, Win 7 (ultimate) 64
(screen resolution - 1600 x 900 - steam instaled on c:/steam/)

Thanks for the help. =)