My System Restore Points disappeared yesterday. I found this from a Google search
"If no free disk space on monitored system drive or on any of the available non-system drives exists, System Restore will purge restore points consistently across all monitored partitions to free disk space. If the free disk space falls below 50 MB on any monitored partition, System Restore will stop monitoring and suspend.
Note: Some users have reported that using the Real Player One utility has deleted restore points. Please review your System Event Viewer log for System Restore events for a volume error event. For more information, please visit the Microsoft.Public.WindowsXP.perform_maintain and Microsoft.Public.WindowsXP.help_and_support."
Sure enough there was a volume error event so it sounds as if Real Player may have been the culprit.