There are many programs for the task, Rename Master is good and it's freeware.
http://www.snapfiles.com/get/renamemaster.html (500kb)
Or if you can live with having files like this:
filename
filename (1)
filename (2)
filename (3)
filename (4)
..then Windows can do the job, just mark all the files on your drive you want to rename, press F2, type in new name and press enter.