Ahh, I fixed it. I was able to get into DOS by pressing F8 and edited system.ini so that shell=C:/Windows/Explorer.exe, things are back to normal. I'm still puzzled as to why Litestep itself worked fine, but the Saturn theme messed things up. It just seems implausible that a shell overhaul would work fine, but something as largely-superficial as a theme would go and crash the system. Has this ever happened to anyone else? Should I contact the creator of the Saturn theme and let him know what happened?