HTC Sync is not compatible with Windows 8 yet. This include not being able to debug and develop on your HTC Desire HD mobil and possible most of HTC mobiles. HTC One, Sensation, Magic and more. Eventhough you have enabled USB debugging.
It’s the USB driver which is missing (or not working correctly) for Windows 8. Google USB Driver nor HTC own driver is not compatible with your HTC mobile. The Google USB driver is available via the SDK and HTC’s driver is via HTC Sync here: http://www.htc.com/www/help/. At some point they will (hopefully) update so it’ll work in Windows 8. You can test if it’s connected with adb from sdk folder
..\android-sdk\platform-tools and this command in command prompt
If you’re up for it, you can create a virtual machine with Windows 7 or previous and connect to your HTC that way. You can of course also if available use a Linux or a Mac.
Check out this xdadevelopers forum post – Windows 8 HTC drivers ADB