I have an 8600GT. Video used to be dim and washed out in all DirectShow applications. This was because luminance had the TV range (16-235) but my LCD monitor connected using DVI uses the PC range (0-255). The only workarounds were making the codec output RGB (a small and tolerable performance penalty), using Haali's Video Renderer (requiring MPC and resulting in "Macrovision Fail" with DVDs in Vista) or messing with contrast and brightness in NVIDIA Control Panel (which are difficult to set properly). All of these workarounds had to be disabled for TV out because it used TV levels.
Nvidia did not seem to care about this driver bug. People had been complaining about for a long time and the workaround they created a long time ago didn't work anymore. It was very disappointing and quite amazing that Nvidia wasn't fixing this bug. It was also weird to think that many people must be putting up with such bad video quality.
Now my problem is finally solved with driver 175.16 for 32 bit Vista from EVGA! Blacks in video are totally black on my LCD monitor connected via DVI. Levels also seem correct on a TV via composite out, which means the driver properly uses PC levels on a PC monitor and TV levels on a TV.
[ content adapted from my posts on the NVIDIA Forums ]