Hope it works for you, maybe this can get stickied so we dont get more questions about fastboot on windows 8,1.
As you all may now, Microsoft pushed the new driver stack in Windows 8.1 so ALL (not only USB 3.0) devices must provide a BOS descriptor. Sony ADB and Fastboot doesn't, so Windows fails to enumerate the device which is reported in Device Manager as error code 43.
This also may help if you have a USB drive or mouse and it stopped working after you upgraded from Windows 8 to Windows 8.1.
Microsoft said that the fix will come in February but for people who don't want to wait I prepaired a dirty solution:
proof - see attachment.
* Your warranty is now void.
* I am not responsible for bricked devices, dead SD cards,
* thermonuclear war, or you getting fired because the alarm app failed. Please
* do some research if you have any concerns about features included in this ROM
* before flashing it! YOU are choosing to make these modifications, and if
* you point the finger at me for messing up your device, I will laugh at you.
I repeat this is a REALLY DIRTY solution that may harm your system, personal data, hardware and/or cat.
Here are the steps:
0. Have a Linux or Windows LiveCD just in case something goes wrong
1. Download and extract files from attachment
2. Navigate to the folder corresponding to your system version (x86 = 32-bit, x64 = 64 bit)
3. Right-click franken-usb.bat and click "Run as Administrator"
5. Wait for it to finish and reboot the system. If everything went fine, you will boot your system successfully
6. Install ADB/Fastboot drivers just like on Windows 8
7. Connect your phone in Fastboot mode to your USB 2.0 port and check if it works
8. Enjoy and wait for official fix
How does it work:
It's not any kind of magic - it just replaces Windows 8.1's usbhub.sys and usbport.sys with Windows 8's ones.
Old files don't check for BOS descriptor so Fastboot can work.
Script updated - it should now actually copy the files but please double check if files were copied to C:\Windows\system32\Drivers!
Hope it helps you!