My suggestion is this:
Buy a router, most routers come with 4 or more 10/100 lan ports also.
One will cost you about $40.00 or $50.00
Then you'll need some CAT5 Ethernet Cable. You connect the laptop to the router via the cat5 cable, and the pc to the router the same way. You then connect your router to your DSL modem. You really shouldn't have to configure anything, just plug them in boot up, and you'll probably be good to go. The router and your PC's might take some configuring which we can easily help you with.
For a few extra bucks, you can buy a wireless router. With that setup you could wire in the PC with the CAT5 cable just like above, but buy a wireless card for the laptop. Then you could use the internet connection / file sharing on the laptop without wires (which is kindof the point of a laptop to begin with)
You could set up a network using Windows XP's "internet connection sharing" tools, with no router, but I really don't recommend this. Setup can be a pain and one computer has to be on all the time to share the internet connection. And especially since you have a laptop, it would really make no sense.