Vista Ultimate - System Backup - No Multiple Restore Poinnts

I have Vista Ultimate and used the System Backup feature to create a Backup
5/7/2007. My friends at Dell punched a hole in the OS yesterday and so I need
to re-image the drive. Before doing so, I made another System Backup incase
something goes wrong.... Now the only restore point for System Backups is
today 7/13/2007. What happened to my other restore point? Note: 400GB
external HDD is being used to back up onto. The only thing on it was the
5/7/2007 System Backup (e.g. 320GB free) and I do not have any partitions on
the external drive and I do not have a dual set up with XP.
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JRG