mike486, welcome to the forum!
I don't know the reasons, but System Restore can't be relied upon on Windows 7 like it can be with XP. You can allocate all of the disc space you want, but sometimes restore points "disappears".
The only sure fire way of restoring your computer is by backing up. There's a handful of 100% free backup programs on the market, such as Macrium Reflect, Easeus Todo, Paragon, Comodo and others. All that you need is a backup drive, either one preassembled, or a spare drive can be placed in a enclosure that usually cost less than $25.
The only other alternative that I know of to replace System Restore is an app called Comodo Time Machine. It takes "snapshots" of your system at various points, and prevents accidental (or intentional) deletion of protected files, as long as the app is installed.
Comodo Time Machine can be found at:
http://www.comodo.com/home/data-stor...a-recovery.php
Hope that it helps!
Cat