How do you set the username that Mercurial uses for commits?

When I commit something in Mercurial like this:

hg commit -m "username question"

I see this output:

No username found, using 'WindowsVistaAdmin@ChunkyMonkey' instead

ChunkyMonkey is my Windows machine name and obviously WindowsVistaAdmin is the user that I am signed in as on this machine.

How can I set the username to something more respectable, or, at least, more concise?