Navigate to Control Panel > System and Maintenance > System and in the left pane, click "System Protection". Here you will se a list of hard drives with checkboxes by each, the checkbox indicates that System Restore is enabled for that volume. To turn it off, just uncheck the volume where you want it disabled and confirm you want to turn off System Protection. Turning it back on is just as easy as checking the box again.

System restore is a valuable feature of Vista that I don't recommend turning off, if you want to free the space that is consumed by the System Restore (Previous Versions) feature, I suggest reducing the space allocated to it without actually disabling the feature.