I got here via Google trying to solve this problem myself, so I hope someone here can help.
I have an MPEG file that was of reasonable quality that I wanted to burn to DVD. After I upgraded Nero 6 to Nero 6 Reloaded I started Nero Vision 3 to add chapter breaks to the video file. In the preview window the video quality had dropped severely. Artefacts were now highly visible, and the whole colour palette and contrast seemed out. I thought it was just due to the preview, so I ignored it.
However when I tried to watch the video file afterwards on my computer, I found that the quality was still terrible, in any program I tried to watch it in (Windows Media Player, WinDVD, WinAMP and Nero's own DVD player).
I assume Nero 6 Reloaded has messed with the codecs or some such, and I have no idea how to rectify the problem.
Any help would be great. Thanks in advance.