Let me say beforehand that I am a lifetime lurker. I would not be posting here if I hadn't already spent at least two days scouring the web for an answer.
My HTC Desire HD cannot connect to HTC Sync, but it can connect as a storage device, and can be seen by adb. Whenever it connects while in hboot/fastboot, it will flicker "USB" a few times before Windows tell me it's not recognized (Device failed enumeration code 43). This includes using the RUU wizard from HTC. That will just make it stick on "RUU" in the bootloader.
I have tried every driver download suggested on this site. I have switched USB cables three times (all htc cables). I have used two seperate computers (Windows 8 and Windows 8.1).
I'll admit I'm a nooby specifically to Androids. All I want to do is flash new ROMs/root my device. As far as I know this is step one, and I can't go any further until I've unlocked my bootloader.
EDIT: SOLVED
HTC Desire HD drivers really hates Windows 8 and 8.1. Installed Windows 7 32-bit and it worked just fine.
