Preventing a program from starting
First, you need to download and install Free Security Soft: Startup Manager.
Permanently prevent a program from starting
To do this simply right click on the item you want to delete and choose delete. Alternativly you can just select an item and pres the Delete button or use the key combination CTRL+D
Note that only the item is deleted and not the actual program. You'll still find it on your harddisk safe and well. It just won't boot anymore on startup.
Temporarily prevent a program from starting
This is a handy option if you want to keep a program from starting up, but still want to keep it in the item list. So you don't have to add it again after having deleted it. You might have noticed before that before every item in the list there is a checkbox. To keep an item from starting up just click on it. When the checkboxed is unchecked the item won't startup while booting unless you check it again.