Try this utility:

http://www.bulkrenameutility.co.uk/Download.php

Alternatively, do a dir > rename.bat, then import the file into Excel and built your own BAT file. Each line will have to be renamed to run a command to ren(ame) the old file name into the new one. Won't take more than 10 minutes to build that batch file. When done in excel, save as plain text and run it. Be sure to run it on a copy of the files, just in case...