The only one I know off hand for the MAC O/S is ITEGO VIRUS BARRIER. It's also known that there are few direct trojan's out and about that affect Mac's though there are some around that affect them via documents though not seriously, probably because Mac's are not as numerous as PC's so not worth attacking heavily.
I don't know what browser you might be using but i,d have a quick look at it's home page setting and also that the option, 'Enable Third Party Browser Extentions' is set too disabled, though if it's already in the systems this will only stop new browser attachments gaining a foothold.
By the way, what is the POP-UP you keep getting, i.e. what is it advertising.