Looking at your dxdiag, your graphics drivers are Driver Version: 7.15.0011.6375 (English); this would seem to be an old version. Have you checked with your manufacturer or Nvidia to see if there are updated drivers for your graphics card ? Please see this article in Knowledge Base
Knowledge Base > Second Life Info > Solution Finder > Technical Issue > How do I get the latest drivers for my graphics card? Topic #: 4051-3884
http://secondlife.com/support/?questionID=3884

(Please note Nvidia's Driver Installation Hints

Nvidia says "When an NVIDIA display driver update is performed, the new driver will remove the previous drivers during the installation processor. Sometimes it is not successful in removing the older display drivers which can lead to display issues later on as a result of conflicting drivers. As a basic practice, it is recommended that you first uninstall the older display drivers before updating to a newer version."

Driver Installation Hints

• "Download Accelerator" utilities should be disabled when downloading any drivers.
• Do not run virus protection software in the background while installing the drivers. This prevents the driver from configuring itself properly.
  http://www.nvidia.com/object/driver_installation_hints.html )

If you still have this issue with RC 1.20.9 please go to this main issue for Nvidia drivers VWR-6733 , add your full computer specs (Help > About Second Life) , crash log etc and vote.

Thank you

This sounds similar to VWR-6715 (1.19.1.4 viewer, Vista Ultimate SP1 & very similar GeForce 8800 card) This issue is waiting on a response from the reporter.

Some things I learnt along the way...

A stable PC browsing the Internet <> a stable PC running 3D games.

I used a utility called "FurMark" which uses Open GL to draw a large rotating fur coloured ring. Yes that's wierd but it does a good job at exercising the PC + graphics card and it also tests the Open GL drivers at least in a limited way. I suggest anyone having stability problems first proves that their PC is stable by running this for 1+ hours. If the PC doesn't survive this test without crashing then you can rule out SL and you have a repeatable test to diagnose issues. (I know - 1+ hours away from SL. But remember, you have to sleep, the computer doesn't. )

When 175.16 drivers were first released the English version appeared to install but didn't actually update the driver. You can check by openning the NVIDIA control panel and clicking "System Information" in the bottom left corner. Solution is to re-download the driver and reinstall. (They appear to have replaced the English version with a corrected International version.)

Diagnosing an unstable system is a nightmare because you have to keep swapping components until the problem goes away. I bought a 800W power supply for £25 on Ebay which I guess was relatively cheap. (I don't want to think about how many Lindon Dolars or I might change my mind.)

And for anyone still having problems - hope they are fixed soon.

Some things I learnt along the way...

A stable PC browsing the Internet <> a stable PC running 3D games.

I used a utility called "FurMark" which uses Open GL to draw a large rotating fur coloured ring. Yes that's wierd but it does a good job at exercising the PC + graphics card and it also tests the Open GL drivers at least in a limited way. I suggest anyone having stability problems first proves that their PC is stable by running this for 1+ hours. If the PC doesn't survive this test without crashing then you can rule out SL and you have a repeatable test to diagnose issues. (I know - 1+ hours away from SL. But remember, you have to sleep, the computer doesn't. )

When 175.16 drivers were first released the English version appeared to install but didn't actually update the driver. You can check by openning the NVIDIA control panel and clicking "System Information" in the bottom left corner. Solution is to re-download the driver and reinstall. (They appear to have replaced the English version with a corrected International version.)

Diagnosing an unstable system is a nightmare because you have to keep swapping components until the problem goes away. I bought a 800W power supply for £25 on Ebay which I guess was relatively cheap. (I don't want to think about how many Lindon Dolars or I might change my mind.)

And for anyone still having problems - hope they are fixed soon.