put this in a dos batch file and run it

@ECHO OFF
SETLOCAL enabledelayedexpansion
FOR /l %%i IN (1,1,1000) DO (
        SET newname=000%%i
        SET newname=!newname:~-4!
        IF EXIST a!newname!.xls (
                ECHO REN a!newname!.xls b!newname!.xls
                REN a!newname!.xls b!newname!.xls
        )
)