|1. How did you partition the hard drive? |
2. What is the drive geometry i.e. Primary / Extended partitions etc?
3. Which partition did you install XP in?
4. Have you set your BIOS so that the hard drive is identified as a bootable device in the boot order?
5. Is the hard drive IDE / SATA? If IDE which IDE interface is it connected to on you motherboard?
6. Did you have a non bootable CD in your optical drive as you were booting your PC?
Without knowing more details it would seem that either your hard drive is not listed in the bootable devices in BIOS or you have not partitioned it correctly.
☺ When everything else fails, read the instructions.