You would think that installing the drivers required to develop on an Android phone would be easy to find. Not quite. Each vendor is tasked with creating their own implementation of the ADB driver, and each vendor happens to place it in a different obscure location. HTC packages their driver on the phone, with the SD card you more than likely have inadvertently formatted in the time since you purchased your phone.
The solution is to download the new package from HTC’s site, however for those of us who don’t want to download a very large installed for <1MB of drivers, I have located a zip file containing only the relevant files: (32-bit | 64-bit)
Happy hacking!