Router Woes

I'm helping my father-in-law hook up two computers, one fairly new and another fairly ancient, to share an internet connection using a regular ethernet router plugged into a cable modem. I thought it'd be fairly simple (and maybe it is) but I shouldn't have been so confident, especially if you're working with PC's.

Besides, I'm an Apple person.

In any case, after about an hour I managed to get the router working with the newer PC (for a while there nothing worked) but I'm having problems with the older PC. My brain was about to explode so I resigned myself to work on the matter sometime this week.