After installing a new hard drive, great step by step explanation on ifixit, I installed the OS version 10.4.7 which the computer came with.
Than I installed the OS X Leopard and connected with the time machine to restore the backup files.
This took multiple hours and this morning I got the message restore complete please reboot. which I did but now after rebooting the gray screen turns to a dark gray screen and the message in a black box appears "Restart computer by holding down the button..." Unfortunately everytime I do this I end up with the same screen.
I run disk utilities and repair disk, but can't detect any problems.