rose avegail , it sounds like you are having trouble with your hard drive. Check the computers BIOS and see id your hard drive is still recognized. IF so, I recommend that you get yourself a Live Linux CD and set your boot order to CD first. Place the Linux CD (I prefer Puppy Linux)into your drive and start your computer. See if it boots up. If it does, you can try to reinstall your OS. If that fails, you may have to replace the hard drive. Hope this helps, good luck.
This means your laptop OS has gone bad. Reinstall the OS using windows XP installation CD. Insert Windows bootable CD in your laptop and follow on screen instructions to complete setup. Remove the CD and finish installation this will restart your computer and see if it gives you OS missing error again.