Sharing internet connection from Windows XP using wi-fi router

That's a very weird set up why haven't you just connected the wifi router to the ISP modem with the ethernet cable

"Most fool proof way:

Disable DHCP on the router.
Assign each of the machines and the router a static IP.
Set the gateway address on all of the devices to the XP machine with the Internet connection.
Enable ICS on the XP machine.

Also, you more than likely won't be able to ping the machine due to ICMP being blocked by windows firewall."