No cigar. I don't have any memory-resident antivirus stuff running. I also tried running in Win95 compatibility mode, disabling some services (Apache, Filezilla)... nothing. I tried installing in the default directory (previously I'd installed into c:\PsychonautsDemo), nada.
I really appreciate the help though! As soon as I know for sure I can run the full version on my PC, I'm going to buy the hell out of it.