I have just had 3 separate System Restore failures on my Dimensions 2400, Win XP. After going thru all the motions and rebooting I then get the Error Message: System Restore cannot be completed.  No changes have been made to your computer.  Try another restore point.'  I had picked 3 earlier restore points ranging from 2 to 8 days but all failed to restore.  What can be the reason?  Please advise.