In Vista, SystemRestore is set to delete restore points after roughly 136 years ... yeah, let that sink...
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 you 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.
Otherwise by default, in Vista, SystemRestore is set to delete restore points after roughly 4,294,967,295 seconds or 136 years, which was a mere 90 days in XP ! The idea apparantly, is to delete them only when absolutely necessary due to space constraint!!! But err...136 years !!!
But if you'd like to, you can also change this interval too. Change the value of RPLifeInterval in the same key; a setting of 7,776,000 seconds (or 76a700 in hexadecimal) is equivalent to 90 days.
SOURCE 
Customizing The System Restore Options in Vista. 
