Hey jbahave, welcome to PCHF.
It's is possible.. but not recommened, as the laptop will need to be switched on for the PC to connect, and will also take up some of the laptop resources. For the sake of $25 from e-bay, it's not worth it IMHO.
However, if you did want to do it, you can do providing you have a network cable, and a network port on each PC. The instructions for which are here:
Internet Connection Sharing with Windows XP
EDIT: Marked as Answered.