I did this between my Win2k laptop and an A3000 once or twice. It should be more or less the same for WinXP.
I opened Network and Dialup connctions, and added a new connection. Set it to "Accept incoming connections". Then
set the port to COM1 or whatever com port you're using. The only other thing you have to do is set the username to use, which I *think* is what you have to plug into Miami/Genesis for your username.
As long as you have the "allow access to LAN" thing turned on in your TCP/IP properties (next page from the allowed users page) then you should be able to connect and ping the PC's IP address. In my case I was then able to access my firewall (which is another PC) and get to the internet, but if you're connecting to the PC that has the only internet access, you'll probably have to turn on internet connection sharing or something to get it to act as a gateway. Then again, it might just work at this point. :-D
I just remembered that there was some file I had to change on the Win2k box... it was a file that had a bunch of expect-reply type strings that told Windows how to chat with incoming connections.. Let me know if you have problems and I'll see if I can find it on my laptop.
Hope this helps!