That means that the HDD that your Operating System is installed on is either close to failing or has already failed and needs to be repaired\replaced -- you can try booting forom the windows installation disk and try a repair installation to see if it can fix the problem but chances are it will happen again since the HDD is starting to go bad -- so best to replace it - reinstall windows to the new HDD and then if needed try and get any personal files etc. off of the old HDD by installing it as a second HDD in the system and copying any files that you need before removing and trashing the original.