Unfortunately, An AT case will not fit an ATX mobo unless you make some modifications. But personally, it's not worth it. You can buy el cheapo case from $25.00 to $45.00 and probably be more than happy than modifying an AT case. In short, if you are deciding on modifying an AT system, sometimes you're better off putting together a new one and give the old one to the kids or to someone special to you.