RENAMER.CMD
@echo off
setlocal enableextensions enabledelayedexpansion
for %%i in (*) do (set name=%%i && ren "!name!" "!name:~7!")
endlocal
Explanation:
The setlocal / endlocal stuff just makes sure that the script will work no matter what your default commandline settings are. Those lines are optional if you make sure that command extensions and delayed expansion are enabled by default, or if you start your command prompt using cmd /e:on /v:on.
The for statement is the heart of the thing. It selects every file in the directory using (*). If you only want to change files with certain extensions you can use a different pattern such as (*.avi). Then !name:~7! takes seven characters off the front. set /? lists some of the other manipulations you can perform. Most of the examples use % instead of ! but all string manipulations work with both.
Updated to answer comment
You can perform a % substitution inside a ! substitution.
@echo off
setlocal enableextensions enabledelayedexpansion
set numtrim=7
for %%i in (*) do (set name=%%i && ren "!name!" "!name:~%numtrim%!")
endlocal