Tweaking System Restore is easy. Simply right click on the Computer icon on your desktop or right click the menu item with the same name in the start menu. Select Properties from the drop down menu.

From the left side menu select System Protection.

Select the drive you want want (which will probably be called ‘Local Disk(C:) (System) and click the Configure button.

Adjust the slider down to around 10gb, click Apply then OK and you’ve finished.