I think it might be possible if you used something like pc-anywhere and used ip adresses within your lan to access the computer connected to the dial up line. Then you, in effect, have the desktop of that computer on your laptop screen (transmitted via your wireless setup) and you can access the internet connection. What you won't be able to do using this setup is to access the internet separately. But if only you are going to use one or the other, it should be possible using this kind of setup.
Another possibility is using internet sharing that Microsoft provides. I've never used it, but I'm sure someone here would know whether that would work with your setup.