Something like this should work:
[*]Copy the image files to a temporary folder
[*]Add those copies to a gallery
[*]Sort the gallery the way you want (by dragging the files to their desired position)
[*]Select the files in the gallery and use Autorename.
[*]When you go back to the temporary folder, you can sort it alphabetically to get the same order that you had created in the gallery.
[*]Press Alt-F8 to verify that 'random order' is off in the slideshow settings, and to launch the slideshow.[/list]