The amount of space Windows uses for restore points is a little more complicated than a single percentage value. The Registry includes its own setting for the maximum disk space given to System Restore, and Windows uses whichever amount is larger: the percentage you specify via the System Properties dialog box, or the Registry's maximum value. Any disk space you free up via System Properties won't instantly be used by System Restore; it will be available until a new restore point requires more space than the amount allotted via the percentage value. The percentage and max values tell Windows only when to stop making new restore points.
To lock in your System Restore allocation, open the Registry Editor and navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsNT\CurrentVersion\SystemRestore. Select the SystemRestore icon in the left pane to see several icons appear in the right pane. Do not experiment with just any of these icons! While you an safely change the value of some of them, Microsoft warns that others should not be altered under any circumstances. Fortunately, you can safely edit the values for the DiskPercent and DSMax icons, which control System Restore's disk-space allotments. To change the maximum amount of disk space System Restore will use (providing it's larger than the percentage value), double-click the DSMax icon. In the Edit DWORD Value dialog box, click Decimal so you can see the specified number of megabytes in the 'Value data' box (the default on most systems is '400'). Change this to the desired amount, and click OK.
While you're there, you can also safely edit the DSMin value, which specifies the minimum space System Restore needs to work at all. Normally, if free space on your Windows drive gets too low, System Restore shuts down and makes no restore points until you have at least 200MB of free space. Setting this value determines the amount of disk space at which System Restore will wake up and attempt to start saving restore points again. However, just because System Restore will try to do so, it won't necessarily succeed if the available space is too small. Unfortunately, we know of no method to determine how much space a single restore point will require, so setting this amount too low could render the feature useless. Still, you can fit a lot of system files in 100MB of disk space.
To change this value, double-click the DSMin icon, click Decimal, and enter your desired amount of free disk space (in megabytes) in the 'Value data' box. Click OK .