I am someone who doesn't run any Anti Virus or Firewall software, and never get them. Now perhaps I am lucky, but most of the time if you're careful with what you click on then you're not likely to get anything. If you're at all unsure though, I say get McAfee. Norton is a waste of your time because in my experiences on other PCs, it'll notice you have the virus, but is unable to do anything about it, whereas many free ones can fix it. That says to me that the program just sucks!

Mozilla Firefox is essential, the new 0.9 version is way more compatible I've found, so I've now switched to using it permanently.