I had Virtual Box 5.0.16 installed.

The dev platform installer (build 33) decided 5.0.16 was bad, uninstalled it, and instead installed 5.0.8 into C:\DeveloperPlatform\virtualbox (where I had had it installed to the default folder, c:\Program Files\Oracle\VirtualBox.

As a result, my existing VB install cannot find the Guest Additions iso image.

I suggest that since the installation of Virtual Box isn't something that is necessarily tied into the DevPlatform, it should be installed into its default folder, c:\Program Files\Oracle\VirtualBox, rather than C:\DeveloperPlatform\virtualbox.

Or if not, the installer could ask the user if they want to use the default folder (for reuse) or install under the DeveloperPlatform folder (for a more self-contained installation footprint).