Do you have the proper drivers installed? I'm not familiar with HTC but I know that for most newer Samsung phones, you need to install Kies for the computer to recognize the phone (that's where the drivers are bundled).
Try going into the Windows Device Manager, delete the driver, reboot, and then re-install. Does your phone give you any kind of error when you plug it in? Does it show any 'USB mode' when you do?