1) you need to go to advance startup option, i am posting for windows 8.1 search how to go to advance startup option for windows 8 in google
a) Press win+ i on the bottom right you will find "change pc settings"
b) click it and choose update and recovery -> recovery -> Advance startup -> Restart now
2) Then in "change pc setting" menu choose Troubleshoot-> Advanced Options ->startup settings-> Restart
3) After restart you will find "startup settings" menu press "7" for "Disable driver signature enforcement" and wait until you pc get restarted.
4) To install drivers go to driver folder in folder most probably you will find it in "C:\Flashtool\drivers" or your flashtool installation location.
5) Double click on flashtools-drivers.exe, in installation menu press next, you will find "select components to installs" menu check flashmode drivers and fastboot drivers click install.
6) You will get some warnings choose install anyway.
You have to follow these six steps if you want to use flashtool on windows 8 or 8.1.