Good response, Ralph, except:
You don't need the RewriteCond
mod_rewrite is NOT the correct tool to use for a simple redirect like this.
Redirect 301 /download/software_old.exe /download/software_new.exe
The reason for this is that mod_alias is a part of Apache's core so it doesn't need to be loaded nor does it have to go off and load the regex engine, too, i.e., it is much faster.
The first thing to learn about mod_rewrite is when NOT to use this POWERFUL tool.