if you want a really powerful rename tool, download Total Commander from
http://www.ghisler.com - it supports probably all you can ever need - leaving a part of the name (eg. if the filename contains number, you can "cut out" this number and use it in the new name), regular expressions, adding a counter, date/time, find-replace, and more... also it enables you to change name and extension separately
IMO it's the best file manager available for windows... for those who don't know it, it also contains ftp client, support for archives - creating, editing, etc., and more... and w/ the new tabbed interface, you can have open as many directories as necessary, and you don't sink in the windows as in the explorer...