System Restore has saved my %#$* a few times, so I let it run on the OS partition, but with less space, so that I have fewer Restore Points to choose from. (I wouldn't want to go back too far anyway!) SR only protects Windows files, not any data or email. Sometimes SR will say "could not restore" because no changes were detected to restore to. SR points are cumulative, so it might be a good idea to delete all but your last one, or shut the service off, then turn it back on to clear it.
If you can be specific about what you want to restore in the registry, we can probably help you do it without having to rely on SR.