The macro & lisp code is straightforward. What's more difficult is describing how to make a button to assign the macro to, andr how to load  a lisp routine.

Macro code:
^C^C_distance;.xy;0;\.xy;0;\

(not sure the semicolon (enter) and backslash (pause) syntax is quite right


Lisp code;

(defun c:dx ()
   (command "_distance" ".xy" "0" pause ".xy" "0" pause)
   (princ)
)

To load the lisp code, you could copy and paste the 4 above lines to the command line. Then run it by typing "dx".

I apolologize in not being more clear in my response :)

You can use the distance command, but exclude the z value when you select the points.
Do this with a filter (type ".xy" when prompted for point), or with "osnapz" variable setting (not sure available in 2007)