Hi there,
U can try these things
1.Lets try to boot the omputer from the OS disk
Put the disk in to the computer and press the power button and hold down the c key untill u get the setup screen
In that screen at the top u have utilities and under that select disk utility and try to repair disk permissions and repair disk
2.If u don have the disk then u can boot the computer in single user mode(Hold apple+s) untill u see some black screen with white text
then type fsck -fy and see what doesn it says
u can refer to this if u need any assistance
http://support.apple.com/kb/TS1417
Good Luck