ScanDisk checks the hard drive for errors. What are errors?  For example, if the computer freezes and you have to turn if off with out shutting down properly, files are left open. ScanDisk finds these files and closes them.  Also, from time to time, files get misreported on the hard disk and ScanDisk will correct the error.  To run ScanDisk (at least every 30 days) follow these instructions:

Windows XP

1. Click on the Start Button.
2. Click on My Computer
3. Right click on C: drive
4. Click on Properties.
5. Click on Tools tab.
6. In the Error-checking box click Check Now.
7. Check the box "Automatically fix file system errors".
8. Click Start.
9. A message will pop up asking if you would like to "Schedule this disk check to occur the next time you restart your computer? Click Yes.
10. Restart your computer so Scandisk can start.

When Scandisk is finished your computer will restart into Windows as normal.

Windows 95/98/ME

1. Double click My Computer on the Desktop.
2. Right-click the C: drive.
3. Click Properties.
4. Click Tools tab.
5. Click the Check Now button.
6. Make sure the Standard type of test and Automatically Fix Errors are checked.
7. Click the Start button
8. Wait, this should only take a few minutes.
9. When finished a message box will pop up and tell you whether your hard drive had any errors on it.

When finished a message window will pop up and tell you whether or not your hard drive had errors.  If it did have errors ScanDisk fixed them.  Click Close and Close again.