Whilst useful it is worth remembering that the Vista System Restore is deliberately designed to allow for several restore points in fact it only deletes them after 136 years. (no joke) In terms of space requirements it will delete older points if the space is required so its self policing.
Warning for dual booters into XP on the same system. When you boot into XP the system wipes out all restore points created by Vista. the reason is XP does not recognise the new restore points format used by XP and assumes they are corrupt so deletes them.