The logbook file (logbook.bin) is saved to C:\Users\<user name>\My Documents\Flight Simulator X Files.. (for Windows 7...XP and Vista path may be different but the files are still saved to the logged in <user name>.)

So if you create and log into Windows with your own user account, the logbook, along with other files like fsx.cfg, saved flights and Granted Rewards, will be yours alone......provided no one else logs in and plays under that user account.

Be sure to give your user account 'administrative rights'.