As new restore points are created, old restore points are deleted on a first
in, first out basis. How old a restore history you have depends upon how
much disk space you allocate as well as how large the restore points might
be. You can't control the size of the restore points because they are based
on the system state at a given time but you can make some changes to
allocation of disk space. Open Control Panel, open System, go to the System
Restore tab, select a drive and click settings. NOTE: the maximum disk
space that can be allocated is 12%, if you are already at 12%, there's
nothing else you can do.
You can try turning off the hibernate feature as that takes a lot of room
and may be bloating the System Restore file. Open Control Panel, go to
Display, go to the Screen Saver tab, click the Power button, go to the
Hibernate tab, and remove the check from Enable Hibernation, click apply and
ok.
NOTE: this will not bring back past points but may increase the number and
depth of history on future restore points.