This error can spell the death of your current Windows installation, but you might be able to fix it if you have a Windows XP installation disc. If you do have this disc, boot from it and choose the Recovery Console option when prompted.

If asked, press [1] to log on to the current Windows installation and press [Enter], then press [Enter] again when prompted for a password.

Once the command prompt appears, type "chkdsk /r" and press [Enter] to let Windows check for errors and see if it can fix them. If it finds and fixes errors, you should be able to boot into Windows once again.