Right so I've downloaded Arma 2 Free to try the game out before buying it at http://www.arma2.com/free and installed it, it asked for a serial number and I signed in with my steam account to retrieve it and that went smoothly, copied the serial number and the installation began.
It finished and I launched the game and opened Multiplayer when I remembered I had to update the game from here http://www.arma2.com/index.php?Itemid=20&option=com_rokdownloads&view=folder〈=en/#free so I did that and returned to Multiplayer.
Now the problem is here: When I try to join ANY server or host one even, I get either an "Invalid CD key" message or "You have been kicked off the game".
I have tried reinstalling the game 3 times each with different directories (C:\Program Files (x86)\Bohemia Interactive\ArmA 2 Free) to no avail.
I don't know if this helps but I'm running windows 7 64-bit and thank you for your time.