Hi, 
First, let me sat that ARMA 2 is such a great game that it lured me back to PC gaming from Xbox.  As context, I started gaming with Falcon 2.0 before there were sound cards, and played OFP when it was first released over a decade ago.  
I purchased Combined Operations via Steam, and was able to download run ARMA 2.   
I then downloaded and ran OA, and go the following message: 
- Bad serial number given in setup 
I then tried to run OA Beta, and got the following error messages: 
- Error compiling pixel shader PSSpecularAlpha:0 
- No entry 'bin\config.bin/CfgInGameUI/MPTable.shadow' 
I have to say that this is the sort of irritating experience that drove me away from PC gaming to the Xbox, "it just works" model. 
---------- Post added at 09:33 ---------- Previous post was at 09:32 ---------- 
re: post above 
This is the machine I'm trying to run it on:  
http://www.amazon.com/gp/product/B009B8EA08/ref=oh_details_o06_s02_i00 
---------- Post added at 09:43 ---------- Previous post was at 09:33 ---------- 
Running Windows 8 - wondering if this makes a difference...