MicroATX PSUs are for slimline cases, which are a lot thinner than most computer towers. An ATX power supply will not fit into any slimline case. Your options are either to get a new case, and move all your components to the new case to use a larger power supply. You will want to check to make sure your motherboard can be moved to a new case without modding the new case, OEMs sometimes use all sorts of proprietary solutions to prevent people from modifying their computers too much. The other option is you get the ATX power supply, but don't put it in your slimline case. You would have to run the system with an open case, or look at drilling holes into the side panel to get the wires in.