I can only guess that the computer has lost its ability to boot from the hard drive and is looking for either a floppy or CD from which to boot. Your inital course of action is to boot from your XP cd, and choose a repair install or use of the Recovery Console.
Note that all data not saved to an external media is now in danger of being lost. Below is a link regarding multiple ways of repairing XP:
Hello! I?ve got a problem starting up XP. A message tells me it can?t find the OP. At the same time the discdrive keeps making the sound of searching for a disk. I?d like to try to put in a backup systemdisc and see if it will start from it.
The problem is that I cant find any instructions how to make one. Do you know?