One issue appeared with Windows 8.1 which I found very annoying : the Wildfire S wasn't recognized anymore in fastboot mode. Not very useful to flash system or recovery images... Usually, the solution was to use Linux, but one cannot (or doesn't want to) install Linux on one's PC :P
So thanks to Auth from 4pda and @niks255 (I'm actually only reposting it here), we now have a way to make these drivers work on Windows. Here's what you need to know :
The actual issue is caused by a lack of BOS descriptor : the Wildfire S doesn't provide any although it has USB > 2.00. Windows 8 accepted it, but not 8.1.
To use this patch, you must have KB296791 (http://support.microsoft.com/kb/2967917/en-us) installed. To check this just go to Control Panel - Programs and features - View installed updates and type KB296791 in the searchbox on the right top corner. If you don't have this update installed, just update your Windows using Windows Update.
Then, download the attached registry key file, Fastboot.reg.txt and rename it to Fastboot.reg.
Open an admin console by right-clicking on the Start button and choosing Console (admin). Then type the regedit command. The registry manager should now open.
Choose File/Import key and import Fastboot.reg.
Done ! It was that simple. Now you can use fastboot whenever you wish