Solution:
On my Windows install, the Mercurial.ini did not get propagated. It also needs a user email added to it.
Take the default Mercurial.ini file found at in the Mercurial executable install directory (C:\Program Files\Mercurial\Mercurial.ini on my machine)
and copy it to your user home dir (C:\Documents and Settings\myName on winXP).
On a Windows 7 install there is no default .ini, you will need to create a new one in C:\Users\myName.
Then edit that .ini file. Find this area. The username needs an email set. It will be blank–add your email name here.
[ui]
; editor used to enter commit logs, etc. Most text editors will work.
editor = notepad
username = userEmail@domain.example
This fixed the problem for me.