Yes, I tried that to no avail. Thanks for the thoughts though!
Further research has revealed that the SQL .exe is a self-extracting archive. I extracted it with WinRAR, which told me the archive was corrupt. I tried re-downloading and re-extracting Express 2005 from MS with the same results.
Just for s#!+s and giggles I tried the .msi inside but I get another installer error from that.