Faulty CD drive ? Faulty/scratched/dirty CD ? CD device not set to master (or slave if on same channel as HDD) ? Faulty Motherboard? CD not at top of bios boot order ? Booting from Incorrect channel ? SATA instead of IDE or vice versa ?
If you post again, please provide more details, like motherboard model, age, memory, type of HDD and CD and how they are connected to the motherboard.