OK, I finally figured out what was wrong.
The problem was twofold and both involved my setup.
The player was having issues using the codecs associated with Media Player Classic. But when I switched to any other DVD player program, including PowerDVD there would be no picture at all.
The problem with PowerDVD was that I had my PC connected to my HDTV via the component inputs. I switched to the HDMI connection and that completely resolved the issue. I get video and at the correct aspect ratio for both Spiderman and Star Trek VI.
So in closing: If you are using Media Player Classic codecs, you need to install PowerDVD. Rifftrax player WILL NOT work correctly with the Media Player Classic codecs. And you need to switch the custom codecs in the DVD settings of the Rifftrax players to the PowerDVD codecs.
Also you need to make sure that your PC is connected to your HDTV via HDMI, DVI or VGA, not component or other TV style connection. Since the PowerDVD codecs will only allow certain outputs to play DVDs.
Huzzah! Great job with the player, guys! Now if I only had more Riff files available for me! (still missing a bunch!)

I'll leave these posts here, in the event that someone else has similar issues.