Well, I thought that building a new computer to replace the current one was a good idea. Of course, computers (especially hardware) can never cooperate when you want them to, and damned if my freshly built computer isn't doing the same thing. First, the relevant specs:

Mobo: Asus A8N-SLI Deluxe
CPU: Athlon 64 3500+
PS: Antec True480
RAM: 1 GB DDR3200 (2 512 sticks)
Vid Card: GeForce 6600 GT PCI-Express
And a hard drive, floppy, 2 CD drives, a CPU fan and a case fan.

Now, here's the problem. I turn on the computer, and I get the requisite BIOS messages, the ubiquitous beep, and such. Everything seems normal, until...about 15-20 seconds later...the computer decides that it's had enough and turns itself off. Try and turn it back on immediately...it turns off in about two seconds. If I let it sit for a couple minutes, it goes back to 15-20 seconds. I just built the thing, so there's no software on the HDD, and it does it regardless of whether or not a boot disk is in. I've unplugged things one by one, and none of them seem to affect its behavior. 15-20 seconds does not seem like long enough for anything to overheat, and besides which, I have had the case open when I turn it on and all the fans are spinning merrily away. The PS only has a 20-pin ATX cable for the motherboard, and I have added a 20-pin to 24-pin adapter to satisfy the motherboard's specifications, to no effect. The PS is 480W and produces 22A on the crucial +12 line, so I doubt there's a lack of power, unless the PS is malfunctioning. Any thoughts?

make sure that everything is correctly plugged in.

Than reset the cmos on your mobo, start up in bios, reset to default, restart and it should work. if not than something isnt correctly done or your psu is out of its use.

Hate saying this but never ever do I use an ATX PSU on a non-ATX board and vise versa. I can see using a 24 to 20 pin adapter but not the otherway. I just read the Antec site and they PS they show working with PCI-Express Mbs with 24pin ATX connector is the True550, not the 480 and below. I would infer from this that a 20pin even with an adapter is not capable of delivering the power to the MB that is required even if the amps are listed correctly or higher than what is needed.