What operating system? I know Windows Server versions offer RAS services from a modem. From that you could probably dial into it and share the modem connection, putting it as the default gateway for local hosts.
I do not know if newer, consumer based versions of Windows have the ability to act as a RAS. I cannot look either because I do not have a single modem anymore.
Hope that points you in the right direction.
Here is a tutorial with XP Pro. You'd just have to share the modem connection and set them as the gateway I believe.