do you have powerdvd 6 or a previous version? If you have 6 then you could try CLEV-2, which is supposed to make it look better, but i'm not so convinced. Other than that you could just select different video enhancements, only if you have 6 i think, you get them by going to configuration then selecting video, theres then a video enhancement box, you could just try them all till one looks better. While your there you can make sure that hardware acceleration is ticked. You can also access it with the little arrow at the left of the player when watching a dvd, it changes the video enhancement in real time.
What resolution is your monitor at? Sometimes your resolution settings can make it so that when watching dvds, your screen might blur or might jitter at the edges.
Finally, also consider this, on pcs, if you don't have a hardware mpeg-2 decoder, then powerdvd will use your graphics card to decode the dvd so you can watch it. This way is not really ideal as they are not designed to decode dvds that well (hence the mpeg-2 decoder, the new generation of geforce 6 cards do however decode better as they have an onbuilt mpeg-2 decoder). This means that you won't get as good a picture on your pc as you would through a standalone, so it may just be this.