I don't know exactly how System Restore works internally, but if it actually saves your restore points in a specific folder or file, you could backup that folder (even to a different drive). Just find out where your "System Restore" files are stored, and copy the entire folder to another location. That way, if you ever need that restore point from 5 months ago, you just have to copy the backup back to it's original location and overwrite the current files.
That last part might get a little tricky. Since it's definitely a system folder, you might have to boot into command prompt to be able to overwrite it.
Let me say though, it's really just a hunch. Look into it before you start overwriting system files. And do let me know if it works. =)