Mobo, Sempron or Power supply

Hello,
i have problem with the new:
AMD Sempron 2200+
MoBo ASRock K7VT6-C (via kt600)
memory JetRam 256Mb
Power Supply 300W Coolink

I have install it with nVidia GeForce2 MX400,
HD MAxtor 40Gb, ...

System has worked about 3 hours perfectly, and then, it was shut down by it self.
Power Led was still in ON state. I was thinking that the system goes to the standBy state. Pressing the buttons on the keyboard, moving the mice, pushing the power button did not help.

I was open the case and wathc what will happen then i press the power button on the power supply and on the case.
Pressing the power button on the case has result in spinning cpu fan shortly and stop. When i was press the power button on the case, cpu fan was again spinning shortly and stop. Power led on the case is in state on when i press power button on the case.
Interesting thing is that the power led it turning off when i keep press the power button on the case for about 2 seconds.
I have allso system with Duron 800Mhz with Asus a7v133m and the power supply from the new system successfully starts the old system. I think that the power supply is OK. Allso, power supply from the old system did not start the new system.

Removing Mobo from the case has the same result like it was in the case.

NVidia VGA is working, HD is working (i checked them on the other system).

Please help! My system is not starting.