Hard to say about the DSL modem having a firewall. I have DSL and the service included a routing modem that is a firewall because I can configure it to be so, It also has logging and other stuff.
I guess you need to contact Verizon and see what type of modem it is.
I also use a software Firewall that monitors all programs-all ports for outgoing traffic, I don't believe XP Firewall does that.