You don't say anything about your system, so it might be difficult to know what the problem might be.
Assuming you use FSX, try to restore the FSX.CFG to default, by removing this file:
%APPDATA%\Microsoft\FSX\FSX.CFG
Then reinstall GSX, because the installer MUST add a line in the FSX.CFG, which will be lost if you reset it to default.
Also, be sure the antivirus is configured as follows:
- Exclude the Addon Manager folder from scanning.
- Exclude the simulator folder from scanning.