Originally Posted by /Ben
1. Computer is booting from a non-bootable source.
2. Computer hard disk drive is not properly setup in BIOS.
3. Corrupt NTLDR and/or NTDETECT.COM file.
4. Misconfiguration with the boot.ini file.
5. Attempting to upgrade from a Windows 95, 98, or ME computer that is using FAT32.
6. New hard disk drive being added.
7. Corrupt boot sector / master boot record.
8. Seriously corrupted version of Windows 2000 or Windows XP.
9. Loose or Faulty IDE/EIDE hard disk drive cable.
10. Failing to enable USB keyboard support in the BIOS.
Originally Posted by sgr215
Although it's already fixed for you for future reference if someone else gets this you can try:
Insert XP cd, enter recovery console, type "fixmbr". I've had this happen when having a lot of different partitions/OS's installed at once and somehow the master boot record gets corrupt.