[Derek1]

I have bough a MA-74GM-S2H and I built it with

CPU: AMD Athlon 64 X2 3800+
Mainboard: Gigabyte GA-MA74GM-S2
Memory: 2 GB Kingston
Hard Drive: Seagate Barracuda IV 40 GB
Video: (Onboard) ATI Radeon 2100
Sound: (Onboard) Realtek ALC 888
CD Rom: TSSTCorp writemaster
Power Supply: Colorsit 500 W

But it doesn't want to start, it does nothing, no fans spin, no leds light...
Then I connected the keyboard (PS/2) and mouse (USB) and I noticed that the lights in both peripherals are blinking constantly and the PSU emits a faint clock-like noise "syncing" with the blinking lights
Finally I researched a bit and I found that my PSU +5VSB line (2.5A MAX) may be protecting itself from a short-circuit or overload.
I tried to put the board out of the tower case but nothing
I tried to disconnect all hardware but nothing.
My PSU works fine with another board.
No smoke or burning smell.
No visible short-circuits.
I returned this motherboard and got the same model: same issue
I'm astonished. What would cause my problem? May be a flaky +5VSB line?
Many thanks.