assuming they're both on xp (although it doesn't matter) you need to network the 2 pc's, you may need a nic (network interface card) or it may already come with inbuilt lan connectors, if using just the 2 pc's then a crossover cable is needed. Then go through the network setup wizzard and make sure you specify internet connection sharing.
There have been a few threads on here about setting up networks so may be worth a search.
I found the best way was to use a router, which gives the added security of a hardware firewall, the connection is slightly different in that you don't use a crossover cable just a straight through patch cable.
I currently have 2 pc's and an x-box connected to the router, it can get a bit laggy, even on BB, when th kids are playing on-line games and I start downloading something, it's normally Daaaaaad! Just waiting for the exchange to go to 1 or 2mb connection now.