I would recommend using Microsoft Security Essentials and Windows Firewall
Windows Firewall is certainly adequate, but MSE certainly is a poor choice. AV-Test uses it as their baseline against which all other software is judged, and hasn't certified MSE in the last 3 evaluations. If you look at testing on AV-Comparatives or Virus Bulletin, MSE gets low marks. Avast and Avira are better freeware options, and NOD32 and Kaspersky still remain my top picks for payware.
Now I know that there is a lot of room for debate about how AV testers rate their products, but even without getting into issues of methodology, when you look at the results across multiple sites, it's consistently clear that there are some top tier products, and then a descending hierarchy of software. I always find it interesting that whenever someone recommends MSE, they don't really ever back up it's effectiveness with test results.
Why does MSE get so many recommendations? I think that essentially: 1. It comes from Microsoft, and we all know we can trust Microsoft. 2. It is pretty light on system resources, so it is likely to run on that old junker that should have been retired years ago. 3. It's free, and we all know there's a group of people that would eat hotdogs 2 weeks past their expiration date if they were free. 4. It isn't intrusive. Users don't need make a lot of decisions to use the software. Updates are automatic and painless, as are scheduled scans. Frankly point 4 is about the only thing MSE really gets right.