OK, so now I see where the confusion was. This is a Fujitsu Celcius W380. You posting 380w makes it look like you were talking about a power supply, not a system model.
Unfortunately, you cannot install any standard form factor motherboard into that case because it was only designed to be used with BTX form factor motherboards. The holes for the standoffs will be in the wrong place, the motherboard dimensions are likely different AND the alignment of the rear I/O panel will not match. If you want a different motherboard you will need a different case and probably a different power supply as well. This is not new or unexpected. These OEM systems are specifically designed to discourage users from using products not supplied by the manufacturer and the best way to do that is to use non standard parts. Plus, with BTX being a technology that ended back in 2006 for the most part, it's likely these parts are so old there would be very little sense in using them anyhow. Looks like that probably has a 1st gen i5 in it, which is definitely not capable enough for anything these days short of the most basic office applications or internet browsing. Those can be equally accomplished with a modern 35 dollar CPU if that is all you need to do, so no matter what the intended purpose for this machine it seems like an upgrade should be part of the plan for the entire platform AND the chassis.