I'll give you two options -
If you want a headache, but enjoy having a huge amount of options at your disposal, then you will like PFrank, named after the programs author - (Peters Flexible Renaming Kit.  Not P. Frank. 

) - 
http://www3.telus.net/pfrank/index.html
If you want it a little cleaner, and have less options at your disposal, then use ReNamer (I have no idea how they came up with that name) - 
http://www.redbrick.dcu.ie/~den4b/