I posted this over at XDA, too. Since 4.3 has it's own thread on the main HTC One board here, I debated on whether or not to post this there, or to post it on it's own thread. But I got so excited about discovering this, I figured, "screw it!" and posted this on it's own, since this was an issue for a LOT of people, it seems. I know...I Googled the hell out of this! I actually put off rooting for months, since the phone made me happy as it was, but soon that old flashing bug bit me. It was time to root! But one problem stood in my way: I upgraded my main computer to Windows 8.1 RTM. And when I put my phone into fastboot to get the unlock code we need to unlock the bootloader, disaster struck. Windows would NOT recognize the phone in fastboot mode. I Googled the error I got, "device failed enumeration". And I tried all the solutions that I came across: installing various drivers, uninstalling and reinstalling the device, uninstalling and resinstalling the root usb hub thru Device Manager, using an virtual machine running Ubuntu...nothing worked. I have a crappy netbook that dual boots Ubuntu and Windows 7, but I wanted to figure out how to make this work with Windows 8.1. Then we got the update today to 4.3, and with it a new HBoot. I wondered if this would make a difference in this situation. So I went into fastboot and connected my phone to the computer. And wouldn't you know, the computer began to install drivers! Before I knew it, the phone was recognized by Windows 8.1, in fastboot! I am now rooted, folks! Not s-off yet, but that will come! I am a happy Panda!