just take it back to them, they can reload it fine. I don't think they will even care. he's right, you don't have the original Autoexec.bat and Config.sys files, but they do. so no sweat. all they would have to do is copy them to your boot drive and BAM, same output.
so, go into the shop with a flash drive and copy the Autoexec.bat and Config.sys files! there you go. then copy them back to your laptop boot hdd root directory.