Run Steam as Administrator (right-click on the Steam icon and click on "Run As Administrator").
Install Arma 2.
Run Arma 2 once.
Install Operation Arrowhead.
Run Operation Arrowhead through Steam (Battleye and Arma 2 Army of the Czech Republic DLC will now install automatically). You shouldn't have any problems and should have OA, Arma 2 and ACR content available.
Exit CO, install both remaining DLCs (British Armed Forces and Private Military Company) and exit Steam.
OPTIONAL (if you need beta patch and want to play DayZ or any other mod):
Download and install DayZ Commander: http://www.dayzcommander.com/
Run DayZ Commander and in its upper right corner, click on 'Versions'.
Get the latest version for Arma 2 and install it. Press 'Check Now' and if it still says 'out of date', do it again until you are up to date.
a. If you receive "Wrong CD key error" then please ignore it - the beta patch should install.
b. You can download and install beta patches from here:
http://www.arma2.com/beta-patch.php
c. You can check your version if you open Arma 2 OA folder/Expansions/beta and run Arma 2 OA beta exe
Get the latest version of DayZ via DayZ Commander.
You should be able to play any DayZ server without problems from DayZ Commander's server list. Please note that the original Arma 2/Operation Arrowhead content will be unavailable while DayZ is active.