Use o PowerShell e verifique se o seu $ files var contém todos os arquivos
#Append -recurse if you want to go recursively, you can also adjust the filter - it gets a list of File objects that match the criteria you set
$files = gci 'C:\WhateverDirectory' -filter '*.*' -File
#This loops through all of the files in $files and renames them.
$files | % {Rename-Item $_ -newname $_.Name.ToLower()}