Open an elevated command prompt by clicking the Windows Orb (Start) > All Programs > Accessories and right-click Command Prompt, then ‘Run as Administrator’.

To display the current parameters type (or copy and paste) “vssadmin list shadowstorage” (not the quotes but the spaces) and press Enter.

To reduce (or increase) the disk space allocated, type (or copy and paste) “vssadmin resize shadowstorage /on=c: /for=c: /maxsize=3GB” (not the quotes but the spaces) and press Enter. This example assumes changing the space on drive C to 3GB. You may find that all the restore points have been removed after resizing, so make sure the computer is behaving itself before reducing the space allocation. Also, make a manual restore point immediately after resizing.