EDIT on 4-1-07: (copied from my post on pg 6)
Since patch 1.2, there's no reason to assume the black screen fix in this thread will work, and certainly no evidence that it does - at least that I've seen*. Patch 1.2 changed all the shader packages. So, if you're on 1.2 and getting this same old problem (and it seems lots of folks are), you can try this fix but I wouldn't hold my breath. Someone else is going to have to come up with some new "fix" to try. (I say "someone else" because I haven't had this issue in many months, so I can't test what might work or not)
*If you're using 1.2 and have had success with ending black screens by swapping out shaderpackage019 and 013, please post.
Another note:
There seems to be some confusion over what exactly the "black screen" problem is. It's not a complete screen blackout or a system hang/lock. It's everything going black except the HUD, with the sound still playing and the game still going on normally. If you can open the command console, type "CAL", and have the problem go away for a few seconds or minutes, it's the black screen. If not, your problem is something else entirely.======================================================================
Original PostAh, the infamous HDR + Nvidia black screen. Some have never had this problem, some have always had it, some get it only after many hours of otherwise trouble-free gaming. Isn't Oblivion great?

I wasn't going to post this at all, because too many times a fix only seems to work for a limited few people, and those that it doesn't work for get all bent out of shape and insulting.
But... I'm going to go ahead and post it in hopes that maybe it will help at least a few folks out. This has worked flawlessly for me - I haven't had an HDR black screen since I did this, with about 15-18 hours of game time played since then. I make no promises it will work for anyone else... if it doesn't, then you're out the 10 minutes it takes to make the changes. If it does, well... I accept payment in donuts....

Please follow all of the following steps in order. They are all important, I promise. Be prepared to reverse any changes if this doesn't fix the problem for you.
The Steps:- Open the Oblivion launcher and enable HDR
- Launch the game, load up a save - any save
- Exit the game (yes, this was for a reason)
- Go to "My Documents/My Games/Oblivion" and open the Oblivion.ini file
- Search for this line: bAllow30Shaders=0
- Change the 0 to 1 (if you haven't already done this in the past)
- Save the file and exit
- While still in the same folder, open the RendererInfo.txt file
- Go to the last line of that file, where it says "Shader Package" and make note of the number there (I'm betting it's a 13, but it might differ)
- Close that file
- Go to "Program Files/Bethesda Softworks/Oblivion/Data/Shaders"
- Find the file shaderpackage0xx.sdp, where "xx" is the number from above... for example, if your shader package in rendererinfo.txt was 13, you'd find the file shaderpackage013.sdp
- Rename that file to something else - I recommend just adding the extension .ORG to it
- Now find the file shaderpackage019.sdp, right click on it and select "Copy"
- "Paste" that file back into this same folder... it should paste as "Copy of shaderpackage019.sdp"
- Rename the new copy to match the name of the file you changed the name of a few steps back, such as "shaderpackage013.sdp"
- Close everything
- Launch Oblivion
- Pray it works
If this doesn't work out for you, first make sure you did
everything exactly as above. If it still doesn't work, just reverse what you did, curse at the Fates, and don't blame me.

To answer a question before it even gets asked: The reason you launch a game with HDR enabled before doing the rest of the steps is because until you do so, the "shader package" listed in rendererinfo.txt may not be accurate - if you have bloom enabled instead, for instance, it will be a different package. Screwing up and replacing the wrong shader package will make your game look really funky... if it works at all.
EDIT:
To keep all the info in one post, I'm including this added step from DagothUhr, which has made this fix work for some people:
QUOTE
1 Make sure you have the latest forceware driver : *91.31*
EDIT: These are no longer the latest drivers. Be sure you use the latest for your card and OS. For WinXP, it's 93.71 (as of March 7, 2007) and for Vista, well, I don't know. 100-point-something probably.
2 Follow the steps *exactly* as luchaire notes,
3 In the Nvidia control panel look for *3d settings* > *Adjust image quality with preview*
4 Choose *Use my preference emphasizing*
5 Set the slider to *balanced*
6 Click *apply*