How to disable Steam from running at start-up

Edit: Evo was faster So yeah, disable any peer-block porgram or antivirus.

I did some searching. Are you using a program called "peer-guardian"? You need to disable this as it blocks steam from connecting.
(Start Peer Guardian , control click on the icon in the Dock. Choose Disable Filters. Quit Peer Guardian. Try Steam again !)

if this is not the problem tell, and I dig some more around for you.