you should use a vpn connection, start vpn from vpn->pptp , create a user list, on client with xp instead ppoe yo should create vpn connection when is asking for ip you need to enter exteriour ip form your pfsense (or can be made with dns if is dinamyc) after enter user and pasw , create new rule from firewall on pfsense to start internet sharing trough vpn or other other options,i've done this and is working fine, good luck