Whether you need to install new software depends, of course, upon what you've already got. However there's one program which should be on everyone's computer because it's the best picture viewing program that's available and it's also great for manipulating images (including batch conversion and batch renames). It's Irfanview, which is available completely free here:
http://www.irfanview.com/
Here's how to use Irfanview for batch renames:
Go to File > Batch Conversion/Rename
Click to place a dot next to 'Batch rename'
Change the 'Look in' field to show the folder where the images which you want to rename are stored.
Click 'Add all' if you intend to rename every image in the folder. Otherwise click on the relevant images, while holding the Ctrl key down, and then click 'Add'.
Use the 'Browse' button [or click 'Use current ('look in') directory] to choose where the renamed images will go.
Determine a suitable naming pattern for your renamed files. If you enter 'picture###' your files will be renamed (using the default settings) as picture001, picture002, etc. For 4-digit suffixes enter 'picture####'. For advanced settings, click 'Options'.
Click 'Start batch'
Chris