do this only if u are comfortable with the registry :
Normally Windows uses up to 15 percent of a drive’s available space to save restore points. In XP, the option to change this figure was configurable from the System Restore tab of the System Properties dialog box. But sadly this option is no longer offered in Vista.
But u can do so as follows :
Open regedit and go to
HKLM\SOFTWARE\Microsoft\WindowsNT\CurrentVersion\
SystemRestore\Cfg Double-click the DiskPercent.
The default is f (in hexadecimal) which is 15 in decimal. For making it say 10% type 'a'
Now again, how frequently you'd like to have your SystemRestore points auto-created can also b configured as follows :
Open regedit and go to
HKLM\SOFTWARE\Microsoft\WindowsNT\CurrentVersion
\SystemRestore
You can change the value RPGlobalInterval from its default setting of 86,400 seconds (24 hours) to, say, 43200 (a8c0) if u want it created every 12 hrs.
As and how your new restore points get created, the older ones get deleted due to the availble space contstraint.
Source :
Customizing The System Restore Options.
http://www.winvistaclub.com/tips.html