This tweak is a great way to keep certain users on the system from launching certain apps. For example, your kid shouldn't be launching AIM since he got caught abusing it last week. Or you've got a community computer and you don't trust certain folks to leave your settings alone in Photoshop. Whatever your reasoning, wouldn't it be nice to know that Joe and Bob can't launch that app, even when you wander off?
Check out this WinGuides article for the steps. Make sure you read it carefully, as it relates to making both key and value registry edits. And never, ever edit your registry without making a nice backup first!
Keep in mind that this tweak will only apply to the current user logged in as you perform the tweak, which can be quite inconvenient if you're working on the down low or don't know their user/pass. As long as you're an admin, check out Daniel Petri's page for a tutorial on editing the registry for other users.