You want to use a ADHOC mode wifi card in a pc as a gateway. Yes that is posable, I have nerver done that; and windows is a pain to set up as a gateway. And you should note that not all wifi cards are able to enter adhoc mode.
Edit: You can use a proxy server under windows to set it up as a gateway. Dont know of any off hand, its been a few years since I have done any thing with a proxy under windows.
JP