1 -> how to leave ip address blank on the windows xp machine :
A : You need a dhcp server on the ubuntu machine. The windows machine has something called a dhcp client which gets its ip address automatically from a dhcp server.
Here's how to install dhcp3-server :
then you can check the 'get my address automatically' box on the windows machine and check that each machine can ping each other.
2-> how to share internet connection on the ubuntu machine :
A : you need the ubuntu machine to forward IP packets to the windows machine.
here's how to forward ip packets :
sudo iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE
sudo echo 1 > /proc/sys/net/ipv4/ip_forward
All apologies for my very approximative english.