Do you want file-sharing between the Mac and the PC, or just internet access from both?
If just internet access is needed, then all you'd have to do is define the default gateway and any DNS servers (you can call your ISP about both of those) on the computers (though I have very little experience with Mac networking, I'll bet whatever you'd be looking for would be in the Networking Control Panel).
However, IIRC, you'll need a seprate program to interface between the Mac and the PC, and I seem to remeber seeing something like that selling for about $100, once (though I wouldn't doubt being able to find some freeware for it, search for something like "PC Mac Networking File-Sharing" at
www.google.com, hmm... after about 30 seconds i found
http://wcic.cioe.com/~galanti/ which tell of someone else's experiences with networking a PC and a Mac [but it looks like he was using a Win95 box, dunno what OS you have], and then there's
http://www.miramar.com/ which offers a program called PC MACLAN, which looks like it'd might be a possible answer).
Well, I hope this helped...