Why do I need both anti virus and firewall for a more complete protection
Firewall protection will stop hackers from getting into your computer or network, and so will stop viruses as well. However you can pick up a virus in different ways such as from a CD or USB, from another computer on the network, etc. You can also download a virus, maybe you downloaded some free software from a website which contained a virus.
For this reason you need virus protection software as well. If a virus does manage to bypass your Firewall protection because you downloaded a game, or something off the web, or picked a virus from somewhere else, then this is where your virus protection software comes into affect, and will detect viruses on your computer.
For the same reason, this is why you would require spyware protection software as well, so that it will detect and remove spyware installed on your system. However usually an anti virus software will also protect you from spyware as well.
This is where an internet security suite can be the ideal solution for the majority of users. If you don't have a virus protection software, firewall protection, spam filter or a spyware protection, then an Internet Security Suite would be the best choice to go for.
In a nutshell you would require firewall protection to stop hackers and malicious threats from attacking you computer, and if you have caught something malicious, then a virus protection product would detect this and remove the threat from your system. If your looking for a virus protection software then take a look at my virus protection guide to find out which one you should purchase, or take a look at my firewall protection guide for a guide on which firewall to get. Or take a look at my internet security suite guide for a full all in one computer protection package.