Please run the files attached to the bottom of this page: http://www.digitalcombatsimulator.com/en/faq/activation_wiki/
(Solution is to wipe out activation data from Windows registry. It will make game to activate as if it was never installed on computer before.)
After you should start activation again.
Best regards and many happy landings !
Konstantin "MotorEAST" Kharin
Eagle Dynamics IT Team