One of the more common types of errors made in assembling a new system is not using the case stand-offs correctly. This can result in a short circuit from the bottom of the mobo to the case, causing exactly the symptoms you cite.
Stand-offs are small (usually metal) support blocks. The common type are about ½" long, with a threaded end sticking out that screws into a hole in the case panel. The other end has a threaded hole, and you put a screw through a mounting hole in the mobo and into that hole in the stand-off.
The REALLY important point with these is that the mobo is designed to be connected to case ground at each of its mounting holes (via the screw through the hole into the stand-off), but nowhere else. For mobo support, there should be a stand-off under each of the mobo's mounting holes. But more importantly, there MUST NOT be a stand-off under the mobo anywhere there is NOT a mounting hole in the mobo. That is what causes grounding at the wrong points.
Cases usually come with several stand-offs pre-mounted on the back mounting plate. But they also are made for several different types of mobos, and the location of mounting holes in mobos can be different, so the case itself has several unused holes for stand-offs. The key thing is, you must examine the mobo mounting hole locations and compare that to the pre-installed stand-off locations. If there are ANY stand-offs that do NOT align with a mobo mounting hole, remove them. Then, if there are any mounting holes in the mobo that do not have a stand-off under them, see if you can install one in a matching pre-threaded hole in the case's back plate. Obviously, to do this you will have to remove the mobo from the case so you can see all the stand-offs. When you are sure there is a complete match with NO stand-off under a place without a mobo mounting hole, then you finish fitting the mobo into the case and screw it down.
I'll mention two other things that can cause similar issues, but these are less common. One is in connecting outputs from the PSU. Most PSU's have a cable that contains 2 or 3 4-pin Molex power output connectors on it. These are about ¾" wide with 4 round holes in a straight line and two corners beveled off. They are used to provide power to older IDE devices and to some optical drives and a few other accessories. On the end of that same set of wires is a different smaller connector, also with 4 holes. It is supposed to be used as the power supply to a 3½" floppy disk drive which many people do not have, so it is not used. BUT some people look at that 4-hole connector and assume it is supposed to plug into a 4-pin fan header on the mobo. If you have done that, DISCONNECT IT immediately. Having that plugged into the mobo forces power into the fan header and causes a major circuit problem that can disable your board.
This last is a really odd one that happened to me. After I has installed the mobo into the case properly and was inspecting fit etc. I saw a problem at the back plate that goes where all the mobo connectors are exposed on the back of the case. The plate has little springy fingers that are supposed to fit around the outside of each connector body to provide a grounding connection from connector body to case. But I had one that ended up with a "finger" inside the connector and touching a contact inside the connector. I had to remove the mobo and re-install carefully so all the "fingers" were outside as they are supposed to be.