I don't even think virus scanning software exists for the 770.
I honestly wouldn't worry about it. Standard rules apply, don't run stuff you wouldn't trust, be aware of what you are downloading. It can't get a standard Windows virus, it would have to have been specifically developed for the Linux+ARM platform which is far less common and therefore not worth the work. It is probable that Opera has a flaw or two in it on the 770, but there really isn't the market base for someone to invest the time into finding a flaw, and exploiting it. That said, someone could write a proof of concept virus, but I wouldn't worry about it.