|I think you can with an ad-hoc wireless network, I can't say for sure as I've never tried to.|
However, no matter what you do, somewhere in there you need NAT. Windows has ICS built into it. ICS performs NAT. So, I would suspect that while you can setup an ad-hoc wireless network to establish a LAN between two computers......you would have to have NAT in some form in order for the second computer to access the internet.
So, you would likely need to setup ICS on the interface with the WAN connection in order to provide NAT (and therefore, internet access) to the second computer.
I've said it before, I'll say it again, a wireless SOHO router would be a lot simpler solution in the long run. If you can't afford the $40 to $50 for a SOHO router then get reading up on ICS in the Windows help files and online in order to find out if you even can use ICS with an ad-hoc wireless network.