Just some suggestions of possible issues:
Is the XP patched?
Is the firewall on?
Is the computer itself hooked directly to the cable modem without a router?
AFAIK that combination could be vulnerable.
Patched - Nope - Next order of business is SP 2.
Firewall - Nope
Router - Nope. Only one box in the house.
I am thinking that Charter’s security will be sufficient. Do you think I should turn off their firewall (if I can) and use either the Windows one of perhaps Zone Alarm.
I want to make this as clean and simple for her as I can.