I've been having the same issue, with the air.pbo file when installing the 1.05 patch. I also downloaded my copy from Direct2Drive. I've tried installing on Windows 7 64bit, and also Windows XP 64bit. I bought Windows XP 64bit just for this game, to see if that could fix it. It's quite sad actually. I haven't had a date in years.
Anyway, let me know if anyone has found a remedy for this issue. Even with this problem, Arma II is still the best game I've ever played. Modern Warfare 2 made me vomit after playing Arma II. Have you ever played a game so horrifyingly boring you projectile vomited? If you want to try it, just play Modern Warfare 2.
I've forgotten what the subject is. Oh, yeah. I can't install the patch and play online anymore. If this isn't fixed soon, then I will most likely have to hang myself.