A great program called Lupas Rename allows you to rename multiple files, be it photos, documents, or anything else easily and very quickly. This howto will show how to use it and what it can do.
To use it navigate to the directory where you want to rename your files.
Select the files you want to rename and take a look at the options. You can change the name, the extension, and you can autonumber them. For example I will rename the files to autonumber before the filename
and I will call them Lupas.
Finally, press Rename and you are done!