Vista's System Restore is set to erase restore points after 136 years :)

Windows uses up to 15 percent of hard drive’s available space to save restore points. In Windows XP, the option to change this figure was configurable from the ‘System Restore’ tab of the ‘System Properties’ dialog box. But this option is no longer available in Vista.

Steps To Set Up System Restore

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 available space constraint.
Otherwise by default, in Vista, System Restore 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 apparently, 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.
To create System Restore Points on the fly, try System Restore Point Creator. Using this utility, you can create System Restore Points in one click. Works on Vista! To change the frequency of the System Restore Points formation, without using the Registry Editor, you can download CSRF Utility.

Enjoy It;)