Problem on WindowsXP (likely will happen on all Win installs), first time using Mercurial. I found the answer in an inobvious place so I'm asking/answering the question myself so others don't have to search like I did.
First time using Mercurial on machine.
Add new repoz:
c:\bla\>hg add
no problem.
Next, commit:
c:\bla\hg commit
error:
abort: no username supplied (see "hg help config")