HOW: First install ICS on Windows 98SE, the PC will be 192.168.0.1 and set the OS X machine's IP to anything in the private IP range 192.168.x.2 - 192.168.x.254. Under System Prefs -> Built-in Ethernet, set the following:
Configure: Manually
IP Address: As stated above
Subnet Mask: 255.255.255.0
Router: 192.168.0.1
DNS: 192.168.0.1
I paste the DNS in when I am ready to connect and delete it when I am done to cut down on the time OS X takes to shutdown. There's some involved permanent solution but this works for me.
[Editor's note: I have not tested this at home as my connection is shared via a router, but the basic premise is the same. Simply point the OS X box at the PC which is essentially acting as the router. Installing ICS is left as an exercise for the reader; this is, after all, a Mac-biased website!]

