Install firewall and antivirus on linux?

While running Linux from disc do I need to download and install firewall and antivirus?

Well, there have been a great many answers, some good and some bad. I like the answer where someone said "you don't need an antivirus and you might need a firewall", but that's not the whole story, either.

As with any computer running any OS (Operating System), you really need to be careful about what you download or run from the Internet. This is not just limited to executables, either; websites with Flash content or Javascript are also dangerous depending on the nature of the site. So, yes, you don't need an antivirus program in Linux if you're responsible about what you download / run. To the person that said that viruses "don't exist in Linux", you're completely wrong; viruses exist in all OS's, but they are MORE of a problem on Windows-based PCs. If you're looking to use Linux for an extended period of time, then I would either install an AV program or I would be especially careful about what I do on this PC to avoid AV problems.

Really, the firewall is much, MUCH more important than the AV, but the question should be "why" you need a firewall, not if. Ubuntu is good about not starting up with too many unnecessary or vulnerable services running; other Linux OS's like Gentoo start with sshd (Secure Shell service), for instance, that could be vulnerable. So, check and see what services are running and disable any that you don't need. Then, to increase the security on your ports with stateful packet filtering, you would have a couple options.

There are a number of firewall products for Linux, one of which was Firestarter, if I remember right. It has a graphical interface, so it's pleasant to the average user, and should be sufficient for your needs. If you're looking to delve deeper into Linux and learn how powerful command line can be, then you'll want to read up on iptables; I configure our Linux firewalls at work with iptables and it has a great number of options. Since it does have a reasonable learning curve, though, I'd give Firestarter a try.

Really, the only question you should ask yourself when wondering how secure your PC needs to be is "what kind of communication will occur to / from my system, and do I need any of those?". It takes a lot of reading and understanding about an OS to do security the right way... for the average person, a decent graphical firewall and some common sense should suffice, though.