Freeware!
Portable!
And a tiny 53.5kb!
http://www.ezyware.com/BatchRenamer.html
With this free tiny bit (53 Kilobytes) of software a user can rename and change any number of filenames, extensions and attributes at once. No messy installing.
All results are shown in the destination filenames before the renaming process begins. Another options for the selection of upper or lower case renaming is also provided.
Source filenames can also be sorted by date, time and name; ready to be renamed in the selected order. Created destination filenames can be replace or altered before the rename process begins"