As usual, it appears everyone is just recommending their preference.
If your router has a firewall then Windows Firewall is certainly good for a firewall that you don't have to "personalize". If you are happy and feel comfortable with Zone Alarm then decide for yourself which you want to use.
You can try different anti-virus programs but Avira is a good one. I would also recommend Avast! 5 Free if you want to try another vendor. With your computer practices, I believe, a free one is enough (There isn't much evidence that a paid one is better than a free one, anyway).
If you want another layer that, I believe,
shouldn't conflict with your set-up and will need very little intervention then you should look into Sandboxie.