The ACAD.PGP (AutoCAD Program Parameters) loads with every AutoCAD session. This file defines keyboard aliases for AutoCAD commands and Shell commands. It typically loads from "C:\Documents and Settings\<username>\Application Data\Autodesk\AutoCAD 2006\R16.2\enu\Support\acad.pgp", where <username> is your login name.
What if you don't like the default keyboard aliases as shipped with AutoCAD? Say you want to change "C" to be COPY rather than CIRCLE. You can modify the PGP file with your favorite text editor, like Notepad, or you can use the Express Tool ALIASEDIT to modify the default aliases:
Pull down menu: Express > Tools > Command Alias Editor...
On the Command Aliases tab, scroll down to C (CIRCLE) and click the Edit button (figure 1.) Enter COPY, or scroll to the COPY command, and click OK (figure 2.) Save and let AutoCAD reload the ACAD.PGP when prompted. Now, entering C from the keyboard executes command COPY instead of CIRCLE.