How do I transfer audible books from my old computer (vista) to new computer with Windows 8.1?

The Audible files should be on your old computer in the C:\Users\username\Music\iTunes Media\Audiobooks folder, though it is possible that specific settings in iTunes and in the Audible Download Manager could have left them in Audible's default download location (C:\Users\Public\Documents\Audible\Downloads). If you can't find the files in either location, you should be able to redownload them from your Audible library - the only exception being any prior purchases that are not longer available from Audible.