First thing I want to ask is, 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.
Windows XP users
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:\
Now that this is the "simple" fix I will give you reality. 9 times out of 10 no one has this damn disk lol. Also when it comes time to enter the admin password most people dont know it let alone even remember they set one lol. Well I hope you can figure it out. Good luck.