Possible causes:
1. Your computer is booting from a non-bootable source.
2. The HDD is not properly setup in BIOS.
3. Corrupt NTLDR and/or NTDETECT.COM file.
4. Misconfiguration with the boot.ini file.
5. Corrupt boot sector / master boot record.
6. Seriously corrupted version of Windows 2000 or Windows XP.
7. Loose or Faulty IDE/EIDE hard disk drive cable.
Instructions on how to fix that can be found
here.