I just built a pc, so i turned the power on and everything worked fine the bios flash screen showed up but right after that it random shuts off. I can still continue to power it on but the same thing happens it shuts off at the biod flash screen. Any help

Where did the HD come from that you used? Was it from the old machine w/ the OS already loaded or is it new and you still have to install it? If the OS is already loaded, try F8 for a boot menu. If you can't do that, then try a boot disk.
Also, try disconnecting everything but the HD in case the floppy or CDROM is shorting.

and check the cpu fan too. is it running? This link may help you too.

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May be a weak power supply.

Assuming you have installed and connected everything else properly and you have the proper ram installed that is actually compatible with your mboard ........

Make sure you have a three wire fan connected to the proper 3 pin header on the mboard for the cpu fan. Many recent mboards have bioses that have default or built in settings that will shut down the system within a very short time if no rpm, or not enough rpm, is detected by the mboard hardware monitoring for that input.
Or, if the cpu fan is connected properly, but it is designed to spin slower than average, some hardware monitoring on mboards cannot interpret the lower rpm properly and will see it as 0 rpm, or as spinning too slow.

Or, rarely, there are several different possible types of 3 wire fan outputs that can be interpreted by the hardware monitoring - some fan outputs cannot be interpreted properly by some mboards and it sees 0 rpm - you have to change the three wire fan to another type.

Other things?
- if you are using a used PS, it may not have enough capacity, especially if it is an el-cheapo brand.
- your PS may be faulty - this is COMMON.