Suppose that I create a wireless ad-hoc network in Windows 7 on a machine A. Suppose that the machine is within radio range of another machine B and B has connected to the ad-hoc network. B can ping A. Suppose that another machine C is within the range of B but not in the range of A and connected to the same ad-hoc network. Can C ping to A via B?
Nope, not in an ad-hoc (IBSS) network. What you're talking about (C and A are in the same BSS but not in range of each other) is called the "hidden node problem", and it's what the "Intra-BSS Relay" service of an Access Point (AP) eliminates. But IBSS networks are AP-less networks, so they can suffer the hidden node problem. All nodes in an IBSS must be within radio range of all other nodes. Thus, the radius of an IBSS network is half that of a conventional AP-based network.
Most Wi-Fi chipsets can be put into AP mode just as easily as they can be put into IBSS mode. On Mac OS X, if you use Internet Sharing to share your Internet connection from, say, Ethernet to the Wi-Fi interface, it puts the Wi-Fi card into full AP mode, not just IBSS mode. Perhaps Windows 7 can do the same thing?