In a nutshell, it's called ICS. Here's a buncha links.
http://www.google.com/search?hl=en&safe=off&q=SHARE+DIAL+UP+CONNECTION+ICS
I have one computer that is windows xp with aol dial up and a modem. the other computer is windows xp with no internet and no modem now i am trying to figure ou how am i going to connect my computer that has no modem to the one that has the modem and internet

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic