What you have is most certainly NOT overkill (all should remain on), if anything you need to consider a third party firewall.
The Windows XP's firewall is better than no firewall but, it lulls you into a false sense of protection, it doesn't provide outbound protection. Whilst the windows XP firewall is usually good at keeping your ports stealthed (hidden) it provides no outbound protection and you should consider a third party firewall.
Any malware that manages to get past your defences will have free reign to connect to the internet to either download more of the same, pass your personal data (sensitive or otherwise, user names, passwords, keylogger retrieved data, etc.) or open a backdoor to your computer, so outbound protection is essential.
- There are many freeware firewalls such as, Comodo, PCTools Firewall Plus, Jetico, etc. - Zone Alarm free works fine with avast and has a reasonably friendly user interface, however, the free version is becoming bloated with trial ware and is also crippled as far as outbound protection goes.
See
http://www.matousec.com/projects/windows-personal-firewall-analysis/leak-tests-results.php later set of results
