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The deletion of restore points may be caused by any one of the following by-design conditions:
You run out of disk space on the system drive or on any one of the available non-system drives, and System Restore stops responding and stops monitoring your system. This behavior causes the system to delete all restore points in an attempt to free up disk space.
You are running low on disk space, but not so low that System Restore stops performing. System Restore deletes some of the restore points, but not all of them. This occurs because System Restore uses a First In First Out (FIFO) process to decrease the size of the data store to approximately 75 percent of its maximum size when the data store reaches approximately 90 percent of its maximum size. The maximum size may be the default size, or it may be set by the user. System Restore performs this deletion regardless of how much disk space you have.
A restore point reaches an age of 90 days. The restore point is then deleted because 90 days is the default time to live.
In other words: If you have enough disk space, restore points are kept for a maximum of 90 days. But if you're running out of disk space, or if the reserved space limit is being reached, restore points will be deleted earlier.
Also note that these two pages describe different operating systems – Windows Vista and Windows XP/ME respectively (see Applies to) – therefore their information cannot be contradictory. (Vista uses a different implementation and does not limit the shadow copy age.)
how do we edit the allowed age of a system restore point?
For Windows XP, the settings are stored in Registry at
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\SystemRestore. The maximum age of a restore point is kept in the
RPLifeInterval value, which defaults to 7776000 seconds.
Windows Vista does not have a maximum age limit for shadow copies.