Vista System has rapidly decreasing hard drive free space for no apparent reason. (System Restore Issue)

Resolution :

Windows Vista System Restore is a function of the Volume Shadow Copy Service (VSS) By default System restore is allocated a minimum of 15% of disk volume for restore points. however, we’ve seen system have much larger limits to include the full size of the drive. In Vista, viewing the drive space allocated to System Restore and adjusting the amount is done via the command line. To see how much space system restore is actually taking up you need to use the Volume Shadow Copy Service Administration Tool (vssadmin, for short), which runs from an elevated command prompt(command prompt run as an administrator)

Start >> All Programs >> Accessories >> Right-Click Command Prompt >> Run As Administrator >> Continue (if prompted)

At the Prompt type VSSADMIN LIST SHADOWSTORAGE
Press <Enter>

VSSADMIN RESIZE SHADOWSTORAGE /ON=[Drive letter of Drive] /FOR= [Drive Letter of Drive] /Maxsize= [Maximum Size you desire for system restore in GB]    <ENTER>
Suggested max limit is 10% of the gross size of the drive.  (250 GB drive, set for 25GB.)

E.G. : vssadmin resize shadowstorage /On=C: /For=C: /Maxsize=2GB        <ENTER>

You will get a confirmation message "Successfully resized the shadow copy storage association"
You can then rerun the VSSADMIN LIST SHADOWSTORAGE command to see the adjusted max System Restore folder size.