I suggest you do not modify the contents of your /library directory. Most things there can be modified per user in their /users/username/library directory. Things inside the startupitems directory do not simply launch on startup, they are instead resources from other programs used during the boot process itself. As mnkeybsness said, you need to go into your System Preferences program and click on Login Items. you have the option of adding and removing programs to run during login. You can also set them to be hidden, eg they do not show up in the dock. This is especially helpful for always-on utilities you don't want cluttering things up.
🙂
pnw