This error is caused by the UAC [(U)ser (A)ccount (C)ontrol] in Windows Vista. To fix this error turn off the UAC as per the following and then start FS One again. You will have to say yes one more time after the UAC is turned off however after this you should no longer see the message.
To turn off the (U)ser (A)ccount (C)ontrol go to Start/Control Panel/User Accounts (Classic View)/Turn User Account Control on or off and uncheck the UAC box. For more, click here.