If you are willing to run one of the computers 24/7 as an internet gateway/router, you can use WinXP's Internet Connection Sharing (a system service that is probably already running). See the help file. Then, you only need to buy some network cable and a hub/switch, and a second NIC for the machine that will be running 24/7 as the gateway. Pros: easy server setup on gateway machine, cheapest solution. Cons: must have the gateway computer on at all times to access the internet from the others, expensive in electricity costs, exposes your machine directly to the Internet and script kiddies.
If you are NOT willing to run one of the computers 24/7 as an internet gateway/router, you can buy one of those home LAN routers. From there, you only need to buy the network cable to connect to the router, and there are step-by-step instructions as to how to do it. SMC, Netgear, Dlink, and Linksys all make routers. Some of the SMCs and Dlinks also include print servers (parallel ports on the routers themselves!) so you can share a printer too without having to run any of the computers. Pros: makes the setup of a home network VERY easy, saves on electricity, some have built-in print servers. Cons: some VPNs need very specialized setup or may not work behind the firewall without placing one computer in DMZ, Netmeeting doesn't receive calls unless the computer is in the DMZ.
I used to run Win98 with ICS, and it was good because it was simple. Win98 has little to be exposed to in terms of risk from script kiddies and hackers even without a fire wall, though I'm sure a concerted effort would've brought it down. But the constant whine of the HD drove me nuts, so we went and bought an SMC Barricade with NAT "firewall" and print server. I have set up two home networks already with this router. Both networks are stable, fast, allow you to put one computer in a DMZ (i.e. exposed ports like it was outside the router), and the print server is great if a little tricky to set up (needs to turn off bidirectional mode and fancy print monitoring features, no problem on our Epson inkjet).