Utraque

I got it! The files on my desktop were not the game files, as I had thought, but were just the install packages for the two games. I went and found the true executables in the Program Files folder (non x86, where I had previously looked), and they worked perfectly! Whew.

I'm having the same issue with my Amazon download from today. I just did a fresh install of ARMA 2, and then Arrowhead afterward. I have not downloaded any versions, free or otherwise, previously. That said, I'm having the exact same symptoms as described by the first poster, and am terribly vexed! Thanks in advance for your time and suggestions.