Hmm.. Are you installing it correctly? Like, are you actually RUNNING the .jar? If so, that's not how you install hacks for Minecraft. You need to download the hack you wish to use, extract it. You should get a jar file. Rename that jar file to 'minecraft' (minecraft.jar).
Then, go to %APPDATA% (in your explorer) and go into the .minecraft folder. Once you're there, go into the /bin/ folder, and paste the minecraft.jar you just renamed there. It'll ask you to do something since 2 files have the same name - make sure you REPLACE. Then, load up Minecraft as usual.