I'm using the latest version of OS X and I don't use an admin password. So every time it asks me for my admin password I just hit enter. I realize that this means someone can come onto my computer and install something, but I'm not worried about that. I am worried about the off chance that some trojan laced program might try to install itself on my machine though. So assuming that I'm not worried about someone installing something on my machine, do I need a password in case some program tries to install itself?