I would just fresh install everything. In fact. I installed everything to my SSD.
The process how I installed everything was install Steam first. Then Arma 2, Arma 2 OA, then if you have the rest of the DLC, download that too.
After you have all those installed, run the games so they can load up the additional files it need for it's setup. Just running Arma 2 OA will load the majority of the files you need. But if you have the Czech Army DLC, you'll want to run that separately when launching Arma 2 OA. The setup process is separate from the other DLC.
Now that you have ran Arma 2 OA and it's good to go, I would download DayZ Launcher as it's the easiest and pretty efficient way to download mods and find good ol' Beatniks.
Once you have DayZ Launcher downloaded, find the Mods tab and download DayZ and DayZ Epoch, it'll take sometime but it will eventually get there.
After the mods have been downloaded, search for Beatniks and launch.
Then boom! You're back in the Epochalypse.