based on my experience with "wired" internet sharing, XP sets itself up as an internet gateway. If you want to share the internet, you need to make sure that the internet is being shared via the right connection (so the "wired" connection i'm assuming) and is set up as a gateway properly, and the other computers are configured to access the internet via the gateway.
Or go buy a router, they're cheap nowadays, $60 or less or even better on sale.
Or go buy a router, they're cheap nowadays, $60 or less or even better on sale.