... by default, Windows Vista uses up to 15 percent of a drive's available space to save restore points? The minimum space is 300 MB. This featured percentage is NO LONGER adjustable in the System Restore tab as it was in Windows XP. If you want to adjust this figure, you will have to edit the registry.
To see how much space your restore points are using, open a command prompt (you have to right click and Run as Administrator and pass UAC) and type:
Vssadmin list shadowstorage
My System Restore is showing:
Used Shadow Copy Storage space: 7.822 GB
Allocated Shadow Copy Storage space: 8.072 GB
Maximum Shadow Copy Storage space: 11.479 GB
By default, System Restore creates restore points every 24 hours and is set to start deleting them after approx. 136 years. In comparison, Windows XP did this every 90 days.