I have a pre-built (factory) Acer Aspire computer. I don't think you'll need the exact name since it's not necessarily a problem with the specific model. I had this computer stock, but then my PSU failed. I put in an identical PSU from my other Acer computer, but now the computer won't start. It will start, but there's nothing showing up on the screen. The case fan, PSU fan and CPU heatsink fan all turn on and my motherboard beeps once loudly telling me it's not a motherboard problem. There's a distinct sound that I've never really "heard" before the new installation, though. My fans and such all power and start running loudly, but around three seconds in it'll all go silent, all you can here is the near-silent hum of all three fans. I don't here the CPU fan running loudly like it usually does when the computer first starts up. I did though take my SATA cables from both my optical drive and HDD, but put them back in. I thought them being switched up might have something to do with it, so I switched them but the problem still persists. Any idea on what's going on?
"My fans and such all power and start running loudly, but around three seconds in it'll all go silent"
Can you compare the sticker on the old ACER power supply with the new one to ensure that they supply the same amount of power.
Is the CPU fan installed and turning ? The BIOS will shutdown if it cannot detect the cpu fan spinning. Sometimes the CPU fan gets blocked by a stray power cable.
Did you knock the heat sink off the processor, then not clean it and reattach with new thermal paste ? (You cannot use the old pad). Sometimes the heat sink gets bumped when you are working in a pc. If the heatsink is lose the processor will thermal out in seconds.
re: "my motherboard beeps once loudly" Try googling the beep codes for your MB BIOS so that you know what the MB is telling you. Acer should have them posted for your model PC. example: If you had an AMI bios then 1 short beep means DRAM refresh failure. You'd pull the memory, try one dim at a time and try to get the system to complete post.