Restore point filling drive to point of no free space

I'm running Vista 5600 and noticed that my 160GB SATA drive had less than 1gb
of free space, eventhough it ony contained 40gb of data.
I looked an found that the drive had 120-130gb of restore point data files
in the "System Volume Infrormation" dir. I was able to delete these and
regain the real estate my drive should have.
System restore is supposed to delete the olderest restore point when the
drive is becomming full? Isn't it?
Any idea why vista needs to restore 1-2 times every day?
Shouldn't it be when ever the system parameters change or a new program is
installed?

Any help would be appreciated.