Hi Mal
personally I don't think you can be too careful - maybe OTT, but I use a router, an Anti Virus and a software firewall. ( the router is primarily for sharing the connection with my son).
My justification is the router NAT function should catch incoming exploits, the software firewall should detect any attempt by a nasty to 'phone home' and the AV should catch any nasties in emails etc. All I can say is that (touch wood!) I have never had any problems with this set up.
hth