Hi,
I would suspect that the boot sector of the hard drive has become corrupted thereby the computer can't find the operating system.
You can try this :-
Boot from the XP / 2000 CDROM and allow the setup to begin. Once the beginning files are copied, you will be asked to Install Windows or use the Recovery Console. Please choose "r" here, to run the recovery console.
You will now type in the following commands when the prompt appears:
chkdsk /f
* This will repair any obvious file system damage, which could complicate things.
diskpart
* This will run the XP disk partition utility. We will be checking to see if your partition looks OK. If you see your partition, and the information diskpart provides appears correct, then that's a good thing. :)
Press [ESC] to exit Diskpart, when everything looks peachy.
fixboot
* We'll write a new boot sector, so the computer knows how to boot the computer
fixmbr
* We'll write a new boot record, so the computer knows how to boot Windows
You should be taken care of. If it still does not work, you may need to delete your partition(s) using diskpart, then recreate the partition(s) (using diskpart, again).
That may recover the windows or it might mean a complete hard drive format and a re-install of windows.
Arnak