Macbook Pro wont boot up after entering correct PW

Hi and hope somebody reads this and can help.
I have a Macbook Pro 15.4. When I turn on the lap and enter the password and fades like it's gonna bootup and then it goes back asking for the password again. i enter it and it does it again. I turn off the lap and I waited like 4 hrs to turn it on again. and it did the same thing. 3x I tried. I even have the Mac OS X Snow Leopard software....but won't work. I thought it would be like Windows were if something like this happens on my PC which it has, all I did was install Windows again and I was happy. I thought the same would go with the Mac and Mac OS X Snow leopard. But No....didn't work. I'm thinking could it be the memory. I have 2 GB DDR3 for memory.
I don't know, that's why I'm here. I need help. I have many pics, music and important docs on the laptop. I would appreciate any help. I'm pretty sure it's not the password because I was able to change the PW twice, but the samething keeps happening and like I said the installation cd won't work either. I've tried the dvd/cd player of the laptop and also a portable dvd/cd player. And the disk won't play. My laptop is a Apple MacBook Pro MB985LL/A 15.4-Inch Laptop and the spec are as follow (Intel Core 2 Duo Processor 2.66GHz, 4GB DDR3 RAM, 320GB 5400RPM Hard Drive, 15.4-Inch Screen, GeForce GT and Apple Mac OS X 10.6.3 Snow Leopard.
Thank you
Joey