The new macros
win32:impersonating-user
and
win32:impersonating-named-pipe-client
allow you to execute code while impersonating a specified user or the current user.
This can be useful in a service process which has sufficient privileges, to perform specific operations with the credentials of some user rather than the system user, or to start an interactive process running as the current user.
LispWorks Release Notes and Installation Guide - 23 Dec 2011