I just purchased ARMA II: Combat Operations and get the Bad Serial Number error shortly after the BI splash screen. I've exited and re-started Steam in administrator. Verified integrity of game cache, which says 1 file is missing and will be downloaded. I've checked the registry to see if the key was there as one other person mentioned, and didn't see one (although I'm not sure the registry path, so it's possible I missed it). I've removed local files and re-installed the game twice. Nothing has worked. Does anyone have any other suggestions to try?