Please Help with power failure

I have a very strange problem and think it is a defective mobo and ask for your advice. I have a system consisting of the Gigabyte GA-K8NPRO-SLI, Aspire 550 Watt, 2GB Corsair XMS, Maxtor 100GB SATA, BFG 6200 and an LG DVD-RW.
All was working fine till suddenly i needed to press the start button 2 or 3 times till the system catch up and start, when it does everything is more than perfect. Today the system won't start, i press the start button and the processor fan start to turn but suddenly the system will shut down. I have no extra cards or devices connected to the mobo but still no luck.
I tried the power supply on my other computer and it worked fine with no issues but the other computer is an AthlonXP and so it uses the 20 pin power connector not the 24 as the new one. Will this has any influence or not.
Any help guys if this is a defective mobo or another way to test the power supply. please help me.

Thanks in advance

Unplug the PSU and disconect it from everything.
Use a wire on its MB main connector to jump the GREEN wire and a BLACK wire.

Plug in the PSU and turn its switch on....if it spins up and then stops it is faulty.

Frys had a sale on Ultra X-Finity 600 W supplys ($49 after rebate...1 per visit) so I get in line 10 times.
The day after installing one it died....I returned all 10 of them!
If people ask me to build a system for them they are going to have to shell out the money for a real PSU...PC power and Cooling.

Z

yes we definitly misread that

In that case, put the aspire back in, and the first thing I would check to check the obvious:

-The 24-pin ATX power **AND** the 4-pin power plug that is near the cpu socket towards the rear right-hand corner of the board, you have to plug it in or no-go for a POST attempt. Also check the 6200 for external power plug but I do not think they have any.
-Check the CPU fan some boards are funny about "no fan" status.
-Clear the CMOS accordingly
-Try and reseat the memory, only one stick in the first dimm, maybe swap them and / or try different dimms. maybe your memory in the old athlon XP pc is DDR you could at least swap out the DDR400 in there and try them one at a time as well?

If nothing still I would start "bare" and pull all the connectors off the mobo & other periphs and only leave CPU&fan, 1 DIMM, GPU, keyboard and power-on header.

If it starts bare, start adding one component at a time and see what is giving you the problem.

If you still have nothing try pulling it out and setting it up on a bench w/ plywood or something non-conductive (wood or laminate is better, no static like some plastics) and go for a caseless power-up. At this point you can try swapping out the mem & cpu with a known working system to isolate the problem (hopefully a friend / neighbor is willing to help)