PC turns on for 0.5 seconds and turns back off

The most common cause of this is a short circuit, often because some part of the mobo (usually on the bottom traces) is touching some part of the case.

When your case arrives it usually has several (up to 9 or 10) stand-offs mounted in threaded holes in the case. These are little metal (often brass) pieces that screw into case holes and offer on their tops threaded holes for the screws that mount the mobo to the standoffs. Now, there are several "standard" mobo layouts with different positions of these mounting holes. On the mobo, you can recognize them because they are through the board and each has a little "flower" array of metal fingers around the hole. Typically there are 9 holes on a full-size mobo - three lines of three each at top, bottom and middle. the metal fingers are there to ensure that the mobo is grounded to the case at these points ONLY. NO other part of the mobo should touch the case.

There are two common ways for this to go wrong. One is: the stand-offs are not pre-installed, and you must do that yourself. But you do not do that, and just try to screw the mobo directly to the case's threaded holes. The result is that the mobo's bottom actually contacts the case in many wrong places. In that case, remove the mobo and install the stand-offs correctly (see below).

The other situation is that one or more of the pre-installed standoffs is in the WRONG location for your mobo and is shorting out a bottom-side trace to the case. You MUST examine your mobo's hole locations and the positions of the stand-offs, and make sure there are NO stand-offs mounted where there is NO hole in the mobo. Ideally, there will be a stand-off for every mobo hole, but one or two missing is acceptable.

Two notes on possible oddities of stand-offs:
(a) some stand-offs are all plastic and actually insulate the mobo from the case. The system then relies totally for mobo Grounding on the sets of power supply wires from the PSU to the mobo. BUT you MUST use those stand-offs to keep the bottom of the mobo separated from the case metal.
(c) I gave seen a few cases where the stand-offs supplied include one or two that have insulating plastic tops on them, rather than threaded holes. These are intended to mount in case holes that do NOT line up with mobo holes. Since they have insulating tops, they can support the mobo from below without causing a short circuit.

I'll mention two other causes of short circuits I have seen. I once found after mounting my mobo in the case that, at the back I/O Shield where mobo connectors are exposed, the shield had tiny springy fingers around most connectors that fit outside the connector's body to make Ground contact. But on one connector the fingers has slid INSIDE and were in danger of shorting out contacts of the connector. I had to remove and re-install the mobo carefully to fix that. The other I have seen is making fan connections. SOME fans are supplied with a dual connector system on their wires - one is a standard 3-pin or 4-pin fan connector that goes to a mobo fan port, and the other is a wide (about ¾" wide) one with 4 pins inside in a line that plugs into a female 4-pin Molex output connector from the PSU. IF you have one of those, you must connect ONLY ONE of those connectors, and not both. If you do both, it feeds PSU power into the mobo's fan port where power should be coming out, not going in!