Files invisible to Windows still play, can't delete.

My refurbished Sansa c140 had 20+ songs on board at delivery. They are invisible to Windows (even with "view hidden files") and can't be deleted. I tried deleting the whole Music folder and resyncing wanted songs to new folder, but the unwanted ones are still there!  How do I get rid of them? I need help too. I've got about 10 files on my Sansa that I added through windows media player. When I was done with them I went onto my computer to delete them but they are "invisable" or something. Help!!!