There are three ways to run multiple websites on the same server. The first is to use different IP addresses, the second is to run them on different ports, and the third is to use host headers by which the server determines which website to serve based on the URL/domain used to access the server.
You can find a good tutorial to set up IIS 6 here:
www.simongibson.com/intranet/iis6. Steps 6, 7 & 8 deal with setting up each of these access methods respectively.