You can search for the rar file if you are confident that you know there won't be any other files with the same name.
dir /b /s C:\Target.rar
Obviously if you know how deep you have to look you can narrow down the search path, and save some time.
If you know it will at least be under C:\Users\%username%
then you can use
dir /b /s C:\Users\%username%\Target.rar