In case you want to disable the FortiClient autostart option (often there's a "Bad parameter" error related when you try this by stopping the FA_scheduler service), here's how to do it (this has worked for me, at least on Windows 8.1):
- shutdown FortiClient (right-click on the taskbar icon - if the shutdown option is disabled that's because you're registered to a FortiGate, so unregister from it first)
- in an elevated command prompt ("Run as Administrator"), run the following command: net stop fortishield
- run msconfig, go to the "Services" tab, uncheck the FortiClient Service Scheduler check box and click "Apply"
- now run services.msc and set the FortiClient Service Scheduler startup type to "Manual" (it should already be on "Disabled")
- restart the system - FortiClient shouldn't start after that