Mark uses a registry cleaner and wants to know what Leo thinks of using them. Leo says that a well designed OS shouldn't need one for maintenance, and a registry cleaner can create more problems than it can cure. He thinks that the promise of fixing the registry to improve performance is simply snake oil, and comes from the older days when Windows just wasn't that great at managing it's resources.
The best thing Mark can do is not use them at all and just make sure his online behavior is guarded. Mark should keep his system up to date, apply all patches, delete all the programs he doesn't need and only get software from reliable sources that don't run in the background.