Hi sookfun,
I've just did all the process described in the Getting Started Guide but I never had to choose between the dpinst-86x and dpinst-amd64. In which step you had to do that? I found those .exe files in the Arduino-1.5.3\drivers folder, but when you have to do the installation process you should select the Arduino-1.5.3\hardware\arduino\x86\tools folder. See the step 3 (Install Drivers) in the Getting Started Guide: Galileo Getting Started - Windows
Regards,
Diego.