Presumably Win XP?

You can store more by increasing the percentage of disk space that is reserved, Start > Programs > Accessories > System Tools > System Restore and the settings are in the left pane. Also, you could keep restore points for a longer period by changing the interval between them, go to Start > Run, type regedit, press Enter and navigate to registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsNT\CurrentVersion\SystemRestore. In RPGlobalInterval it is set to 86400 (seconds) which represents 24 hours. Change it accordingly. It is prudent to create a system restore point before changing the registry.

You can remove all but the last one, Start > All Programs > Accessories > System Tools and run Disk CleanUp. When it has finished click on the More Options Tab and at the bottom select “Remove all but the........”.

You can remove selective restore points in XP Pro. It is much more difficult in XP Home.

In order to keep a specific restore point you would, sensibly, be running XP Pro and would continually have to monitor it to make sure the requested one is not deleted.