Problem with the screen dell inspiron 1520

Hi,

I have problem. When i turn on my laptop dell inspiron 1520 the screen is black form the beginning. When it boots up only the mouse can be seen and some times when i press Alt+Ctrl+delete the screen turns on and it is displayed the desktop  but it stays like that frozen only the mouse cursor moves but can not perform actions. I know that the laptop is working because i use it to share internet and the others laptop can connect to it. This happen when i put a new RAM memory inside. After that i removed the new RAM but the problem still remain. 

I tried holding down the D key and press the power button and it works, the colors were shown. I tried Fn and power up but noting was shown only i could hear sounds and diodes flashing. 

I connected to external monitor and it worked perfectly. I run Fn and power button and all the tests passed. The BIOS is set up as it suppose to be. I even change the CMOS battery(some body suggested) but the problem remains. 

So is it the video card, the screen, the cable? OR? 

Best regards,

Aleksandar