First, you must understand the files to be deleted, make sure if they can be deleted or not. Usually, data files can be e.g. mp3, photos and documents, program files e.g. exe, dll should not be deleted. So files under My documents folder can be deleted but not under Windows and Program files.
You can use Smart marker to help you to filter duplicates by date, folder, file name pattern etc. http://noclone.net/pdt.asp?doc=mark.htm
Files are deleted at your own risk, so always consider moving files to a temporary folder to archive for some time before deletion.