All antiviruses still offer full support for windows xp. I used to use comodo firewall + panda cloud. But I also used comodo antivirus + firewall.
If I were you, I'd choose ZoneAlarm antivirus + firewall.
Zone alarm are faumos for their firewall, they had a good score according to the latest AV-Test. It updates automatically everyday and it has good detection rates...
I use Comodo Internet Security and i am happy with it.It doesnt uses a lot of resources, it has a firewall and a high detection rate.It also has a sandbox and pretty strong HIPS. It has a program called kill switch (more information about it here: http://help.comodo.com/topic-119-1-328-3518-.html ) its smilar to task manager.It also has a virtual kiosk (more information about it here: http://help.comodo.com/topic-72-1-451-4738-The--Virtual... ).It also has a gaming mode.You can also make a rescue disk with it.And i think it is the best free AV.