Try this button (see here if you don't know what to do with the code block):
<?xml version="1.0"?>
<button display="both" label_pos="right">
<label>Rename Explorer Style</label>
<icon1>#duplicate</icon1>
<function type="normal">
<instruction>Rename NUMBER=001 REGEXP PATTERN="^(.*)(\.[^.]+)$" TO="{dlgstring|Enter base name}[#]\2"</instruction>
</function>
</button>
If you press it you will be asked for the base name and then it will rename all of the selected files and folders so that they have whatever you enter followed by a number, then their extension.
The button ignores files/folders which don't have a . in their names and might not work the way you want it to on folders that do have a . in their names. I think to make it work perfectly with those cases would take a bit of VBScript. If you need that then shout and I'll write it (it's fairly simple) but if the button above is good enough then I won't bother. 
