It doesn't really address many aspects of your question (I am nothing if not an 802.11b newbie!), but one of the first things I would do is to go get something like this
. Don't sacrifice your NAT but make your life easier and get it off your Windows box. ( I realize that this may not be an immediate possibility if you have an internal DSL "modem", but still think it would be the better approach to take if you are interested in internal networking flexibilty and expansion possibilities while preserving some basic security).
I don't have the clearest mental picture of your setup and no idea why the Windows box would be denying SMB filesharing unless it is configured somewhere to do so. I gather cable providers have begun blocking SMB/Netbios but didn't know that any specific DSL providers were -- and that wouldn't seem to apply anyway...doesn't shound like you're trying to share to somewhere outside your ISP's networks.
have you thought about fixing the problem by using Windows 2000?
(edit: hmmm, if your considering a WAP, how about one of those combo router/NAT/WAPs?)