I had the exact same problem (with my Gigabyte HD 5870), H246 videos would make my computer BSOD in win7 and just crash the drivers in winXP. Same with youtube videos if I had flash hardware acceleration on.
The problem was that my motherboard (which is like 2-3 years old) had a too old VGA Bios version and the solution is obviously to update it.
Now both my motherboard and HD 5870 are from Gigabyte and this is how I did.
1. Download VGA Tools --@BIOS from: http://www.gigabyte.eu/Support/VGA/Utility_List.aspx
(the app in the top row)
2. Download the latest Bios VGA binary file for your graphics card. Now Gigabyte have their own system for their graphics cards names and mine was "GV-R587UD-1GD". Usually you can find this name on your receipt or on the box somewhere. Alt. you could install the --@BIOS tool and it's display the name for you.
3. When you have the Gigabyte-name for your graphics card goto http://www.gigabyte.com/support-down...ad-center.aspx
and search for the name there.
4. Click your graphics card, Downloads, choose Bios under Download type and download the latest version unless it's a beta version. I used version F8 (on 2010-09-07). When you've downloaded the file, just unpack to your desktop.
5. Install the --@BIOS tool and restart your computer just to be sure.
6. Run the --@BIOS tool, backup your current bios file (mine was just version F3 when latest was F8 :S)
7. Click the Flash button and select the binary file you downloaded in step 3 and let the tool do its job and then restart your computer when it asks you to.
This completely solved the issue for me. I can play all kinds of videos and could turn on flash hardware acceleration again with no more BSOD.