If you have a Windows XP CD available (any version, but it must be a 'proper' Windows CD rather than a recovery CD provided by a large computer manufacturer) then the problem can often be fixed by following these steps:
1. Insert the Windows XP CD in the CD drive. Turn the computer off, wait ten seconds, then turn it on again.
2. Watch for a message stating 'Press any key to start from CD...' or similar. Press a key on the keyboard when this appears. If Windows tries to start as normal, the settings in your system BIOS will need to be changed to allow the system to be started from the CD.
3. A blue screen will appear, and information will flash at the bottom. Wait for a screen which invites you to setup or repair an installation.
4. Press the 'r' key to start the 'Recovery Console'.
5. A black command line screen should appear. You may be asked to choose the installation to work on. This is normally '1' (C:\WINDOWS). Press the relevant number and press enter.
6. You may be asked to enter an administrator password. This could be the password you use to log onto Windows. If no password is set, press enter.
7. Enter the command CHKDSK C: /P /R. This will set Check Disk to run through the drive and 'repair' any faults that it finds. It may take several hours for the process to process. (Note: The 'F' switch does not work from the recovery console.)
8. Type 'EXIT' and press enter to restart the computer and check to see if the problem has been resolved.
Note: If the process stops progressing for more than four hours, there may be severe damage to the hard drive. Turn the computer off by pressing and holding the power button for ten seconds and consult a technician.