Well form what I have noticed on my machine shows no difference in video on XP/Vista/Windows 7.
The one question I have is the poor quality you are talking about is it just video's or in games too? Also what is the player you are using? Do you have the newest Nvidia drivers
, chipset/mobo and sound card drivers? All of those effect video playback and quality. So does disk defragmention, is the video run from the hard drive or over a network, usb or DVD? I know alot of question and no answer but I would like to have more information before saying just anything.