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):

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)
2. in an elevated command prompt ("Run as Administrator"), run the following command: net stop fortishield
3. run msconfig, go to the "Services" tab, uncheck the FortiClient Service Scheduler check box and click "Apply"
4. now run services.msc and set the FortiClient Service Scheduler startup type to "Manual" (it should already be on "Disabled")
5. restart the system -  FortiClient shouldn't start after that