Make sure all registery keys are removed.
If you are not experienced I advice you to use ccleaner to do it for you. Its free software:
http://www.filehippo.com/download_ccleaner/
First make sure to uninstall all Arma software. Than manually remove any folders which are left (so check your installation path).
After the uninstall, reboot your PC (lets just do that for fun and cause its windows).
Start ccleaner, run the registery cleaner. When asked, make the backup before running it!
Reboot PC, re-run ccleaner and the registery cleaner (just to be sure). If it finds nothing re-install the game.