This is a repost of a thread by @izi501 in the Wildfire S forums; I just wanted to help spread the word. Credit goes to Auth from 4PDA and @niks255.
The issue has something to do with the BOS descriptor in Windows 8.1. It has been fixed in update rollup 2967917.
1. Install update rollup 2967917 for Windows 8.1.
- To check if you have the update installed already, go to Control Panel > Programs and Features > View Installed Updates > type "KB2967917" in the search box.2. Download the attached file Fastboot.reg.txt and change the extension to Fastboot.reg.
- If you don't have the update installed, open Windows Update to install it.
3. Right click on the Start button and choose Command Prompt (admin). Then type "regedit" and press enter.
4. Go to File > Import > choose Fastboot.reg.
That should be it; Windows 8.1 should now be able to detect your phone when in fastboot mode!