has the laptop got a network card on the motherboard ?
if not you will need a PCMCIA Network Card, they are available in wireless versions as well as RJ45 network cable connections.
absolute bottom line, to get the laptop on the net as well as your main pc you will need a PCMCIA Network Card (RJ45) a Crossover cable and a PCI network card in your main PC.
but DreamCaster is right when he says a router is the best option, as that way you wont need both pc's on just to get the laptop on the internet - plus Internet Connection Sharing (ICS) on windows realy sucks.
another option for you is client / server, this requires much more hardware, but is modular and infinatly more flexible.
for this you will need a network card in each machine (as with the ICS and router options above) you will also need a pc to act as your server (this is the pc connected directly to the internet).
this pc isnt required to have a high spec, as long as it has a hard drive over 1gb in size and enough free PCI slots to add one or maybe two network cards. I presume your modem connects to your pc with usb? this being the case you need just one network card in your server, but a slightly newer pc, one with onboard usb.
the operating system for your server would be a Linux based firewall/router such as
Smoothwall - its free but you need a cd burner to put it on disk.
connecting all these together is a Network hub, you plug your laptop and pc into that (via the network cables) as well as the the server pc. smoothwall handles the IP address asignments and the firewall (you set it up and manage it via a web browser, so the server pc doesnt even require a monitor, mine doesnt even have a keyboard and mouse attached).