There are two bundles, CMSF and Marines for $25 usd and Marines/Brits/NATO for $40 usd.
Since you have the base game, if it is a BFC version, just get the MBN.
I have Windows 8.1. I switched from my user log on to Admin log on. I right clicked the setup.exe and selected Run as Admin. I still get a popup stating the Windows has blocked it. I click more... and it allows me to run it anyway.
I only have MS Security Essentials for firewall and AV.