Taking as our starting hypotheses:
(1) Windows programs do not run in the Mac OS environment and cannot be converted into such.
(2) You don't want to run Parallels, or Bootcamp, or some kind of virtualization.
(3) You are talking about software, and not talking about specific file types, e.g., doc files.
The only conclusion is:
The "easiest way possible" is to get out your credit card...