Can't start my MacBook, turns off when I release the power button

MacBook Pro early 2008

As the topic I can't start my MacBook. When I press and hold the power button i can hear the CD-ROM drive, fans, sleep mode indicator and my hard drive to start. But when I release the power button everything quits and I hear a quite beep sound from the side where the hard drive sits. The screen is black.

When I try to hold the power button longer (about 5 seconds) the sleep mode indicator start to flash fast and then the computer quits.

The problem started after I was working with logic pro and closed the screen for sleep mode. When I came back I couldn't start it. Lately my computer has been slow and my processor been almost working on maximum.

I have tried lots of things, resetting the SMC, safe mode (don't pass the release button), resetting PRAM, hold down ctrl + command + power, changing the RAM circuits with an old MacBook pro think 2006 model that works.

I thought it could be my harddrive so I tested it in the old MacBook and it worked (but it wanted me to resett to 10.6 or something).

My MacBook is running mountain lion.

Do someone know what the problem is and how to fix it.

Can you some how check what hardware is not working? It's a old computer and I don't want to put lots off money to fixit so I hope it's not the motherboard.

Thanks