The System Restore feature is built into Windows vista and is used to return your computer to an earlier state if you have a system failure or other major problem with your computer. The point of System Restore is to restore your system to a workable state without you having to reinstall the operating system and lose your data files in the process.
System Restore can use up to 15% of a hard drive for its backup files (shadow storage). As hard drives get ever bigger, that becomes a lot of space. The command “vssadmin” can be used to administer settings for System Restore. To control the space allocation, open a command prompt with administrator privileges and enter
vssadmin Resize ShadowStorage /For=C: /On=C: /Maxsize=[n]
For [n] enter the desired size in MB or GB. The units must be stated: for example, “Maxsize=500MB” or “Maxsize=2GB”. The example is for the C: drive.
WARNING! This will delete all your old Restore Points!