For several years I've used BRU, also known as Jim Willsher's Bulk Rename Utility. I haven't posted this before because I thought his great free programs were well known. Since I couldn't recall if I saw it here (at least not for awhile), I thought I would post the links. Don't let the GUI scare you. It's an excellent, powerful, rename utility for files and directories.
http://www.bulkrenameutility.co.uk/Main_Intro.php
He also has a very nice conversion utility.
http://www.jimwillsher.co.uk/Site/Software/UCU/UCU_Intro.php
Gizmo lists BRU as his 3rd File Renamer recommendation in his 46 Best Freeware Utilities list (actually he lists alot more than that).
However, if you read the reviewer's description, it's his renamer of choice: