Open the explorer folder that contains the files and do the following:
(be careful not to include any critical windows system files or folders when applying this technique. back up with system restore first if you're not sure. also, it only makes sense to apply it to files of the same type.)
1. Highlight the files and/or folders to be renamed (remember to hold down the CTRL key to select non-adjacent files); or hold CTRL-A to select all the files and folders in a given folder
2. Let the mouse hover over the file that you wish to be the first in the series
3. Hit the F2 key or Right-click and select "Rename"
4. Type in the new name for the batch of files you've selected (don't forget to type in the extension if needed, for example .mp3 or .jpg)
5. Hit the Enter key to change the names. The files will remain highlighted and should have sequential names like "rename (1).txt" and "rename (2).txt". If you're not satisfied, simply hit F2 or select "rename" again and do it over.