Dead ZT1180 Pavilion laptop

hi all, i'm at my wits end here. My laptop will not power up, when it does actually stay on for more than 3 seconds the display is really dim and all you can see on the display is the hp logo whicj is barely visible. I brought it to a repair shop and they told me it needed a new mother board so i searched the web and found one, we replaced it and when we tried to power it up it did the exact same thing. now i doubt i bought a board that had the exact same problem so i'm guessing the board was good, well i tried replacing the ac adapter, thought maybe it was dying and couldn't send enough juice to power it up, wrong still the same thing, tried replacing the hard drive, the ram the sound card. arghhh..... what is causing this problem. i like this laptop and want to fix it not trash it, anyone know of any possible solutions for this problem i am in need od some serious help here and will take any advice i can get...other that trash the laptop