I found a great walk-through on how to enable file sharing on Windows XP.
Besides this article, there are a number of other great resources for Internet Connection Sharing, among others. Take a look: http://www.practicallynetworked.com/sharing/
I've found file sharing on Windows to be feast or famine: either it works great right off the bat or you run into any number of un-explainable errors with no obvious way forward.
Thankfully there is a whole team in Windows (of which I’m a part) working to improve the file sharing experience for Longhorn. To quote another PM on my team, “our goal for LH should be that it doesn’t take a 10 page technical discussion to get sharing to work.”
I couldn’t agree more.