Yeah, it looks like its the game files themselves that are corrupted. Looks like I'll be buying/downloading another copy from steam. Thanks for your help, though. I was really impressed by how fast my question was answered.

The shareware version of the game works fine. Is there any way I can copy files from the original game folder to the shareware version to make the shareware version have access to all the other episodes, or is my only option to purchase/download another full copy?

I have an old IBM PS/1 PC that had a good number of old games on it, including one of my favorites, Wolf3D. A little while ago, I added the Super add-on pack by downloading it onto a couple floppies and installing it on the PS/1. Now, I have copied the wolf3d folder to my new computer, and am trying …