By default once System Restore reaches 90 percent of the allotted size, System Restore begins to "purge" older Restore Points on a first-in first-out (FIFO) basis until the 50 percent of its allotted size is free.
The minimum size used by System Restore is 200MB. If your Windows drive has fewer than 200MB of free space after you install Windows Me, System Restore is disabled by default and you will have to manually enable System Restore after you have freed up sufficient disk space. If System Restore is disabled due to the low disk notifier, System Restore automatically re-enables itself after sufficient disk space is made available. On logical drives that are less than 4GB, the maximum size used by System Restore is 400MB. On logical drives that are 4GB or larger, the space used for System Restore is approximately 12 percent of the logical drive that Windows Me is installed on.
You can also adjust the size of used by System Restore manually, follow these steps:
Select Start > Settings > Control Panel > System
Select the Performance tab
Click the File System button
On the Hard Disk tab, adjust the System Restore disk space use slider to the preferred size