"Bad serial number given in Setup" Message *NEED HELP*

So, yesterday, I bought the ArmA II + Operation Arrowhead add-on combination package, entitled "ArmA II: Combined Operations" on Steam in the hopes that I could play the Day Z modification. Upon installing the two, I tried to open each one, individually. When I open ArmA II, it will go as far as the BI image, and then it closes the program, leaving a message stating that a "Bad serial number given in Setup". I tried to open Operation Arrowhead, to see if it, too, was acting oddly. OA doesn't even open, and instead leaves a long error message that I did not write down. I decided that the easiest and most simple of actions would be to uninstall and reinstall each program. ArmA II just got done installing, and OA is at about 10%. I tried to reopen ArmA II, but the same problem keeps occurring. I will not install the program a third time, as that is most certainly not the issue here. I expect that when Operation Arrowhead is done installing again, the same problem will arise, leaving me deflated and in need of reassurance, but I do not wish for this predicament to be that incessant for that long. Rather, I would particularly enjoy some assistance from any individual(s) whom have been having the same problem. Thank you for any help you offer, and I hope to see you online soon enough.

Try to run steam and the game as admin.

If it doesnt help, contact support@bistudio.com

And you search next time please. Its been answered so many times already.