so wait, how do any of the other PC's access the internet?

It sounds like all you need to do on the XP machine, is share the LAN with the dial-up using internet connection sharing.

cause you already have the ethernet to USB adapter, and that works as you said at the library, and you have a hub at home (i'll assume you said that on purpose, and it's not a router)

so, the XP machine will become a NAT/DHCP server (a router) when you turn on internet connection sharing, and then anything plugged into the hub alogn with the XP machine will be able to access the dial-ip (assuming the dial up is already connected)