A router makes a great "outer limit" firewall, stopping the internet background radiation of port probes etc., but does not protect against anything that comes along on pages you view.
Unless you can stick to a vast sweep of preventative measures, such as using a safer browser, preventrative filtering etc., then it's a good idea to use at least a free antivirus - and not an awful lot to choose between the big 4.
AVG is popular, but (maybe law of averages), it's easy to find someone who's been let down by it.
Avast 5 seems to be getting good reviews - also seems that, out of all the big 4, it may have the least impact on performance, if a few seconds on the startup bothers you.
And MSE is now at version 2 - with suggestions that although it's an improvement, it's also a fair bit "fatter" than version 1