Excellent features and tiny size make Batch File Renamer one of the best file renamers around.

You can rename entire directories of files, or filter out the ones you don't want to change by their extension. Presets let you convert filename cases instantly, as well as replace characters in the filenames, add a number sequence to the name, and for advanced users you can even run your own command line on source files.

The program runs from a self-contained EXE that takes up a mere 48KB space.

The top half of the program is for selecting the source directory and the file type (.doc, .txt, .jpg, .htm etc...). Directory navigation is accessible from the menubar, but it feels archaic and it's probably easier to copy and paste.

The bottom half of the program is where users can choose what changes they want enacted on their filenames. Presets include case changing, changing the specific filenames to generic naming conventions followed by numbers, and character replacement.

Download this tiny yet useful application from here : Batch File Renamer - Download.com

yeah! really useful

For these works, I use Ant renamer.
This program is bigger than you specified but it had a lot features and very friendly :
antp.be > Software > Ant Renamer