I've done this with Win2k, so it's probably very similar with XP (but Home might be a little different).
What I had to do was make a new network connection.
The first question the wizard asks is the type of connection you want. You have to pick "Accept Incoming Connections".
Then pick the port that you are connecting your Amiga to.
Next I answer "no" to the VPN tunneling question.
Then you have to choose or add a user that will be allowed
to connect.. this is the user and password you use for
Miami.
The rest is pretty straight forward, except for one part which I don't remember the details of any more.. :-(
What I do remember is I had to edit a *.mdm file in Windows somewhere, and change an init string. Something about the way Windows PPP works that doesn't work with Miami out of the box. I found the info on Google at the time.
Once that's done, IIRC I just had to set up Miami for a PPP Connection using MS-Chap for authentication, put in the user name and password from the PC, set it to do a direct connection so it doesn't try to talk to the modem, and that was it.
Clear as mud? :-)
Hope that helps a little, at least!
If you get stuck, let me know and I'll check on the A3000 and see if it's still got that config set up in Miami.