Lots of options on this one:
Cause:
Computer is booting from a non-bootable source.
Computer hard disk drive is not properly setup in BIOS.
Corrupt NTLDR and/or NTDETECT.COM file.
Misconfiguration with the boot.ini file.
Corrupt boot sector / master boot record.
Seriously corrupted version of Windows 2000 or Windows XP.
Computer is booting from a non-bootable source
Many times this error is caused when the computer is attempting to boot from a non-bootable floppy disk or CD-ROM. First verify that no floppy diskette is in the computer, unless you are attempting to boot from a diskette.
If you are attempting to boot from a floppy diskette and are receiving this error message it is likely that the diskette does not have all the necessary files and/or is corrupt.
If you are attempting to install Windows XP or Windows 2000 and are receiving this error message as the computer is booting verify that your computer BIOS has the proper boot settings. For example, if you are attempting to run the install from the CD-ROM make sure the CD-ROM is the first boot device, and not the hard disk drive. Second, when the computer is booting you should receive the below prompt.
Press any key to boot from the CD
When you see this message press any key such as the Enter key immediately, otherwise it will try booting from the hard drive and likely get the NTLDR error again.
Note: If you are not receiving the above message and your BIOS boot options are set properly it's also possible that your CD-ROM drive may not be booting from the CD-ROM properly. Verify the jumpers are set properly on the CD-ROM drive. Additional information about checking the CD-ROM drive connections can be found on document CH000213.
Additional information: This error has also been known to occur when a memory stick is in a card reader and the computer is attempting to boot from it. If you have any type of card reader or flash reader make sure that no memory stick is inside the computer.
Computer hard disk drive is not properly setup in BIOS.
Verify that your computer hard disk drive is properly setup in the BIOS / CMOS setup. Improper settings can cause this error.
Corrupt NTLDR and/or NTDETECT.COM file
Insert the Windows XP bootable CD into the computer.
When prompted to press any key to boot from the CD, press any key.
Once in the Windows XP setup menu press the "R" key to repair Windows.
Log into your Windows installation by pressing the "1" key and pressing enter.
You will then be prompted for your administrator password, enter that password.
Copy the below two files to the root directory of the primary hard disk. In the below example we are copying these files from the CD-ROM drive letter "E". This letter may be different on your computer.
copy e:\i386\ntldr c:\
copy e:\i386\ntdetect.com c:\
Once both of these files have been successfully copied, remove the CD from the computer and reboot.
Misconfiguration with the boot.ini file
Edit the boot.ini on the root directory of the hard disk drive and verify that it is pointing to the correct location of your Windows Operating System and that the partitions are properly defined.
Corrupt boot sector / master boot record.
It's possible your computer's hard disk drive may have a corrupt boot sector and/or master boot record. These can be repaired through the Microsoft Windows Recovery console by running the fixboot and fixmbr commands.