My laptop started taking some time boot and finally stop booting.

If it's powered on battery, when a hit power button the power LED comes on and goes off after 2 seconds. On AC power the battery is charged and hitting power button the LED comes on goes off again and repeats this cycle forever. The display is always black, no light appears.

I opened it up and disconnected everything (disks, CD, keyboard, display, memory, etc). Tried powering the system board with AC power and turning on the LED stays on and after a while the cooling vent starts spinning. Reconnected the display and tried again, but nothing on the screen. Then i inserted one memory module, and it goes back to turning off after 2 seconds with any beep or display initializing. Tried several memory modules and result is always the same, goes off after 2 seconds. Tried a external display but nothing ether.

Is the system board, video card ? What else can i try to figure it out ?