Automatically prefix file name with a number

I need a program to rename a file that has just been dropped into a windows directory with the file number followed by an underscore. e.g. if a directory has 223 files and i drop the file "[login to view URL]" into this directory, i need a program to amend the name of this file to "[login to view URL]". This should be a relatively simple program and shouldn't take someone who understand windows or dos scripting too long. I am open to 1. either clicking an executable in that directory which will then rename the last dropped file, or 2. some sort of job that monitors the directory and updates the file name appropriately.