HTC seemed to want to install Sync in order to get the USB driver. So I downloaded an older driver from here:
http://downloads.unrevoked.com/HTCDriver3.0.0.007.exe
Opening a Command Prompt and running adb devices showed that the USB driver was working.
C:\Program Files\AppInventor\commands-for-Appinventor>adb devices
* daemon not running. starting it now on port 5037 *
* daemon started successfully *
List of devices attached
HT155TJ03448 device
