[Iska]

Примерно так:

Код:

@echo off
setlocal enableextensions enabledelayedexpansion

set sFolder=d:\bik

if exist "%sFolder%\." (
	for /f "delims==" %%i in ('dir /b /a:-d /o:n "%sFolder%\*.arj"') do if not defined sArcName set sArcName=%sFolder%\%%i
	
	if defined sArcName (
		echo Found archive [!sArcName!]
		echo "%sFolder%\arj.exe" x "!sArcName!" "%sFolder%\" -r
	) else (
		echo Not found any *.arj files in [%sFolder%]
	)
) else (
	echo Not found folder [%sFolder%]
)

endlocal
exit /b 0