Answer by wullxz for How to rename 80.000 files at once in Windows

Try Powershell (preinstalled in Windows 7):

Get-Childitem /path/to/your/files | foreach-object { move-item $_ $("._"+ $_.name) }

(tested it in my download-dir.)

Edit: Siim K's code will append an additional ".jpg" to every "._filename.jpg".Remove that last ".jpg" in Siim K's code and you have a short, elegant code to rename your files.