please help - installation issues with DirectX

Hi All I get an error when installing PMDG 737NGX regarding DirectX - Error is " A cabinet file for installation cannot be trusted. Please verify the crytographic services are enabled and thee cabinet file certificate is valid" Is this critical to the installation. After the error is simply push ok. The aircraft is in my FSX at statup and i get the cockpit view etc but as soon as i try move or switch views FSX says it has encountered an error and needs to restart! Any hints? I have a low end speced computer but fly the PMDG747 without hassles. Many thanks, Rory Windowx XP, Core duo 2GHz, 2GB RAM, NVIDIA geforce9300 (258MB)