You don't need to tell your ISP because essentially the second computer will just share the first's internet connection. You will be able to share files and printers if you select that option when you run the network wizard.
You will need a
crossover network cable which essentially is just like a normal cable, but some wires are swapped over. It looks the same as a normal cable essentially. Your local computer shop should be able to help you there, or it's fairly easy to make yourself with a kit from somewhere like Maplins.
On both machines you just need to run the wizard to "set up a home or small office network" which is really simple to follow. You need to run it on both machines, and just do what it says. As I say its quite simple. You can use your Windows XP CD to run the wizard on the second machine just as Choli said.
To be honest, its really simple thanks to the wizards so hopefully you should be up and running in no time!
