Nowadays, unless you visit risky sites that require you to have supurb protection, a free version of an antivirus (again, often MSE because of its compatability with windows) is all you really need.
If you browse the internet at all on both machines, i would highly recommend an antivirus. Defender is not an antivirus and will not protect you as well as one.
Interesting to note: Defender for Win8 IS going to be an antivirus software. MS is playing around with names once more. This software is scheduled to be included with Win8 upon its release.
Tip: some people are saying AVG is doing a lot better. I would stick with it (provided it is working well) until your license runs out. After that, I would switch to the free version of my favorite antivirus. Mine would be MSE because it has very high ratings for malware removal/protection and works great with my system.