I use my PC to connect to the Internet and then browse, send mail etc. via Windows 98 Internet Connection Sharing (ICS). Why? The modem on my PC always connects faster, and more importantly, I have CallWave internet answering machine for which there is no Mac client.
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!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020402001357858