Are you really sure you don't want to just get a wireless router? The way you're trying to set this up, your notebook won't be able to get online unless the main PC is up and running.
Sounds like you're trying to wirelessly hook the the notebook (Does it already have a wireless NIC?) to your main PC (I presume it has a regular NIC that you've connected to the modem?) and use ICS on the main system to provide connectivity for the notebook. Seems like a needlessly complex way to go about this, and the only money you'll save is the difference between the cost of a wireless NIC (USB type?) for the main PC and the cost of a wireless router. And you'll have a network that is LOTS less flexible.
Yes there are USB-connected wireless NICs that you can plug into yoru main PC. I seem to recall reading about people having a fair amount of trouble getting that sort of equipment to work well for Internet Connection Sharing, though it certainly works well enough for connecting a system to a wireless router or AP. You might try searching to see if you can find a pertinent thread.
- prosaic