The download versions appear to be working - I've now beaten episode one! So, unless you did something very creative between the DVD and the download, I'm content to lay this at the feet of SecuRom - I know I've had a problem with them once before. (I had to delete and reinstall some driver I didn't even know I had.)
By the way, Kudos to the music guy. The games great, but the music really is way, way above the bar.