I have vista and can only give these words.......
Vista will only save about 5 restore points in the hdd image, because the restore images carry a TON of information reguarding your installs and updates, i'm currently working with a corrupt file system right now that can crach at any time but still haven't backed up my os in a while. And if you defrag with vista it will auto delete the oldest points first, but has been known that you can go all the way back to when you first update your system to restore the previous, it's kinda weird but i guess thats vista for ya. Wait till Win 7 comes out later this year it will be much better than vista and only uses 1 gb of ram to operate, more less a hyped version of xp pro. Hope this helps!!
Duck