Hi,
Based on my research, I suggest you try the following steps to see if it works.
1.Press "Start Key/Windows Key + C" or move your cursor to top right corner of the screen to activate the Charm Bar and click on "Settings Button" as shown in following figure.
2.The setting panel will open. Click "Change PC Settings" from the bottom as shown.
3.You will have this PC Settings screen. Click on General Tab. and scroll down to the bottom to find "Advance Start up Option" as shown below and click on Restart. This is immediate restart so make sure all programs are closed and saved.
4.Your PC will reboot into Advance Start Up option. And you will see following screen. Now simply press on "Troubleshoot" option.
5..In troubleshoot click on "Advance Options. In advance option select "Start-Up Settings" .
6.Now just press restart button in Start-Up settings as shown below. Your system will reboot in Start-Up Option Mode.
7.After your system is rebooted you will get this following screen. Here to choose options we have to use function keys. Now press "F7" to select "Disable Driver Signature Enforcement" .
Now your PC will boot with Disabled Driver Signature Enforcement and you will be able to install Flashtool/Fastboot drivers.
You can get this drivers from Post #2.
If the issue persists, I think the ADB fast boot related driver had a compatibility problem with Windows 8.1.
Also, I will forward this case to our product team.
Regards,