Reinhardt, it sounds like what you are seeing is happening due to a common problem with HDTVs today. Many of them are designed from the flawed premise that all HD signals that are sent to the TV will be correctly formatted as 16x9, so the TV does not offer any option for reframing the image.
That catch here is that your HD DVD player is upconverting the 4:3 DVD to an HD signal without adding any "windowbox" bars, thus confusing the TV into thinking that it is receiving a widescreen image, which results (as you have seen) in a stretched image.
You don't mention which model of HD DVD player you have, but I think most if not all of them have a setting to force the windowbox bars to be added to 4:3 images, which will fix the problem. Unfortunately it's been a while since I researched it, so I might be wrong.
If all else fails, one thing you can do is to use a second, inexpensive SD DVD player JUST for viewing 4:3 material, since it won't upconvert, and the TV CAN make necessary adjustments when it receives a 4:3 image at 480p.