Are you using the same piece of software to playback your HD content as you use to playback your DVD's?
Or is Media PLayer opening your HD content and PowerDVD (or simlar) playing back your DVD's?
My guess is that your DVD playback SW is doing something whacky with the aspect ratio, perhaps look for a 'maintain aspect ratio' button.
You could try using a single piece of SW to playback both of your formats.. Media PLayer Classic is a small download, can be found here:
http://www.afterdawn.com/software/vi...er_classic.cfm
If you still find you have the issue I am puzzled as media player classic has it's own in-built DVD decoder so would negate any issue with the PowerDVD (alike) decoder being setup wrong.
Last thought.... your HD content may be anamorphic, if this is the case and your DVD is standard 16:9 then I would say that your resolution is set wrong. Ideally it should match your display device exactly.