Need user name, not computer name

[Mark Powell]

How can I determine one's user name?  I have been using the address property
to extract it, but have just learned that this is the computer name, which
IS&T changes when a computer is upgraded.  Ergo, one user can no longer
launch the standalone.  On a related note, I see through the debugger that
there are distinct globals $USERNAME and $COMPUTERNAME, but do not know how
to trace how these globals are set up by the engine.   Perhaps $USERNAME is
what I need to use?  Does this persist across hardware changes?