Assuming you have not added any hardware or made changes to your operating system, your computer is telling you it can't boot from the hard drive. Usually this is because files have become corrupted or the hard disk is failing.
If you have added any drivers or hardware remove it and reboot and see if it fixes the problem.
If the data on your disk is valuable to you, don't continue to use this computer. Remove the hard disk from it and install it on another computer as a slave drive. Then make an image of the drive with an application like Ghost or Acronis Disk Image. This will make an exact copy of the disk and preserve any data you have in its current state.
After you have made your backup, go to the hard disk manufacturer's web site and download their diagnostic utilities (most of the name brands like Seagate and Maxtor have them). Or, use a third party utility to examine the status of your hard disk. If there is an error on the disk it may be correctable using these utilities.
If this does not work you will have to use a file recovery program like Get Data Back to pull your old files off the damaged disk.
The main thing to realize, if you are working with important data, it that you often have one chance to retrieve it and the longer you use the disk the less likely recovery will be. If you make a mistake or use the wrong tool you may worsen the problem and make data recovery impossible. Also, a disk that is failing will worsen the more your attempt to fix it because the recovery process will stress the drive. By making an image of the disk you have a copy of the existing disk, so you can go back, reload the image, and try it again if your first attempt fails.
Note also, you do not have to remove the hard disk to perform these techniques, it is just safer to maintain the integrity of your data. If you boot from a utility disk it is quite possible to image your drive to an external hard disk directly from your laptop. Then you can use the Windows installation disk to load a repair installation. Here is an excellent guide to do this (read the entire article first):
http://www.michaelstevenstech.com/XPrepairinstall.htm
You may want to consider taking your computer to a professional for this if you cannot afford to loose your data.