Captain Stilgar, as the first step please try to set Compatibility Mode with Windows 7 on the both client.exe and client_,exe files (here \SteamApps\common\Naval Action) and reply back with the results.
Also to clarify - do you have Win SP1 installed?
Another workaround is to turn AMD CrossfireX feature off, but as you provided the specs this is must be not your case.