would be to buy a wireless router and plug the cable modem into it.
Then get a wireless network adapter for your computer - either internal PCI or, if you're not keen to open up the case, an external USB model - either type will work well.
Get a PCMCIA card type adapter for the laptop, although a new one may come with a wireless adapter already fitted.
1. Connect the router to the modem.
2. Connect the laptop to the router, using the USB cable that comes with the router.
3. Follow the instructions to set up the router.
4. Install the wireless network adapter software on both the laptop and the PC BEFORE you connect the adapters.
5. Fire up both computers and watch Windows detect the adapters and install the software.
6. To network the computers, run the home networking wizard on the laptop (Windows XP), and when it asks you if you want to make a floppy disk for use on other computers say yes. (always assuming you have a floppy drive in the laptop - if you don't you'll have to skip this).
7. Insert the floppy disk in the PC and start it. Follow the prompts to set up your network.
N.B. - all computers on a home network must have the same workgroup name - accept the default MSHOME unless you have a reason not to.
The router will automatically establish the connection via the modem whenever it's switched on.