|Make sure the power button on the case does not stick inward, such that the power switch is engaged all the time, otherwise the the mboard will shut down in a short time, the time depending on bios defaults or custom settings (they're always under 10 seconds). |
Read the mboard manual and make sure you have all wiring connected to the correct places on the mboard. Do not just go by markings on the mboard beside pins - that can be confusing.
You must install the cpu heat sink on the cpu properly. It should be installed before you mount the mboard in the case so you can see that it's flat on the cpu. There must be a thermal pad, thermal paste, or some other thermal compound between the heatsink and the cpu. Thermal pads are often installed on the heat sink already - you must remove the protective covering from the pad surface before you install the heat sink.
If the heatsink is not flat on the cpu, the cpu will heat up much more quickly, and when it does, the mboard may shut off in a short time.
Make sure the cpu you chose is on the list of supported cpus for your mboard model on the mboard maker's web site.
Make sure the ram you chose is on the list of modules for your mboard model on the ram manufacturer's web site.
Always unplug the computer or otherwise disconnect it from AC power before you fiddle with any connections or components inside the case
You must connect the 3 or 4 wire cpu fan female connector to the proper cpu fan header, otherwise the mboard will shut down in a very short time to protect the cpu from burning out when no rpm is detected.
The same thing can happen if the connector is connected correctly if the cpu fan spins slower than the slowest rpm the bios expects to detect by default.
Your video card probably has at least one if not two sockets a PCI-E 6 or 8 pin connector from the power supply must be connected to.
Your mboard has another smaller socket other than the 24 pin one a connector from the PS must be connected to.