Missing device driver for "Android 1.0" in Device Manager while in HBOOT menu!
Installing the driver from the XDA Dev website should solve this issue.

(There is also a driver and a tutorial on the Unrevoked website. But the this driver didn't work for me. While the driver from the XDA Dev worked every time.)

You essentially need to download the adb_driver_htc.zip file and view the hardware ID of your HTC device (Properties dialog of the device in Device Manager) and make sure it matches the ID in the INF file for this driver. You then need to update the driver for the device (in Device Manager) with the missing driver (yellow triangle with exclamation mark).