Mathew Kirkland has put together a routine that will determine whether the version of AutoCAD installed on a particular machine is 32-bit or 64-bit. This is useful if you manage various machines in a mixed environment, because some third-party routines require different files to be loaded depending on the version.
Included with this routine is a sample LISP file that loads the AutoCAD MDI tab program. This sample uses the 64-bit test function and loads the appropriate ARX file. Instructions for using the 64-bit test and the sample function are included in the comments. You can download the MDI tabs ARX file from Autodesk at http://autodesk.blogs.com/between_the_lines/2009/04/autocad-free-bonus-tool-drawing-tabs-mditabs.html.
(Note that it is possible to run 32-bit versions of AutoCAD on 64-bit hardware. Harry did not test all possible combinations of software, operating system, and hardware.)