Loss of former desktop files

I recently upgraded a windows ME edition computer for a client to Windows XP Pro. He originally told me he had no files to keep. After installing, of course, any desktop files he had were gone. He now says that he had some saves some critical files in a folder on his desktop, but saved them nowhere else! Is there any way to recover these or roll back the upgrade? All files that he had save properly to a c:drive folder are still present. Help!