The situation:
I had powered my PC off using the normal shutdown sequence in Windows. About four days later I returned to turn it on. The light flickers, but then nothing else. There was nothing unusual to the power-off sequence. There is no sign of a power surge during the four-day off period.

This is a system peiced together by myself. opened the case and removed everything but the RAM and the CPU. The problem persists. The problem in more detail:
When the power supply power switch is turned on, the green light on the mother board lights, and stays lit. Upon hitting the power button, the power LED on the case flickers, the fans (CPU and Case) start up but quickly start to slow down. The motherboard LED remains lit. Any attempts to start the PC fail completely until the power to the motherboard is completely drained (disconnect the power until the green LED on the motherboard goes out). This process can be repeated.

The equipment:
ASUS P4V8X-MX (normally I use an alternate video card, but this includes onboard video)
Intel Celeron D 2.8Ghz (or close speed)
1GB Ram
400W Power supply (generic, exact model/number unknown.

The biggest question I have is what would make this system, which has ran faithfully for 6 months start acting this way, and what can be done to fix it?

It must be your power supply, although the green light on the motherboard stays lit, only a small amount of power is required to keep that light on, however to actually boot the computer (CPU, Hard drive) much more power is required. this is quite common by the way, power supplys fail all the time, try testing the computer with a new psu, quite a high chance it will work.

let us know.

A failing or defective power supply can certainly partially work yet fail to let your computer boot completely. This is common. See response 4 in this:
http://www.computing.net/hardware/w...

Thanks for the quick replies. After a new power supply I have my computer back. Thanks!

I spent a little time troubleshooting after the replies and with a voltmeter, found to voltages to be quite low on the 12V leads - well, even on the 5V leads. Unsure of if I was testing the PSU correctly (since there was no load, and I 'turned on' the power supply out of the computer) brought it in to the local shop which verifed that it was not working. It was an El Cheapo brand; I guess you get what you paid for, though I've had other cheap PSU's last several years.

Thanks for your replies.